Thanks to the ID Software webpage for this information.
The Lost Episode: "Keen Dreams"
Your main goal is to collect 12 Boobus Bombs to use against King Boobus Tuber in the last level. There are more than 12 Boobus Bombs available in the game, so you can pass on a few if they are too hard to get.
This first level is fairly easy so that you can get a good feel for the mechanics of play.
There is a free Keen directly overhead at the start, but, it's just out of sight and out of jumping range. So, make your way up the poles and platforms so that you can jump down to get the Keen.
There are several Flower Powers, and 2 Super Flower Power pots to be found in this level. There is a Lollipop in a niche along a cliff face.
The Exit is at the far left, bottom of the area.
Watch out for the Carrot Couriers which will try to push you into harm's way!
The Melon Mines
Located just north of Horseradish Hill, this place is a huge labyrinthine mine complex with many levels, or, floors.
The Exit is to the far right, near the upper levels.
There are several Super Flower Powers, Keens, Boobus Bombs, and a 5000 point Cotton Candy in area. Definitely get the Boobus Bombs! You'll need them to use against King Boobus Tuber.
The hidden room with a free Keen: Upon entering the Mines, travel to the left, negotiating the terrain, until you reach a long, deep vertical shaft. Standing on the edge, jump up and left, grabbing an unseen pole inside the ceiling which will lead up to the room.
The Magic Eyeball: Near the upper right corner of the Mines is a floating eyeball that will give you 4 Keens and 8 Flower Powers if you grab it. It is just a couple levels above the Exit, and not difficult to find from there.
The Grand Prize Chamber: Locate the large open area full of many ledges and ramps. It's a couple of levels below where you first enter. Make your way to the bottom-right corner of the large area, where you will find a long pole which appears to go both up into the rock above and down into the rock below. There are 3 places to go from here:
1. Back up a few steps and go up the short nearby pole to get a key. Get the key and come back down.
2. Climb down the long pole, with the key, in search of the Grand Prize Chamber. Go down to the bottom of the pole, where the passage then goes to the left. Ride the traveling melon cars across the dangerous green sections of railroad track. Jump over the fire pit. Go to the far left side of the passage and climb down the pole there to the next level and travel to the right. A locked door will open with the key and reveal the goodies.
3. Climb up the long pole (mentioned earlier), searching for a Keen, some Cotton Candy, and the Exit. About 2/3rds of the way up the pole, jump off to the left into a hidden chamber. You will find a Keen and some Cotton Candy there. Get them, and go back to the pole, climbing on up to the Exit. Yer outta there!
Located due south of Horseradish Hill, past the bridge, you will find Bridge Bottoms, a place consisting of a sky full of mosquitoes above, a log jam below, and a river below that.
There are 3 exits from this area, 2 on the ground (left and right), and 1 below the water level (to the far right, under water). They all lead out, but the water exit will lead you to a hidden Keen and a Super Flower Power.
A Super Flower Power is on the uppermost-left log in the log jam area.
There is a grouping of 5 Super Flower Powers and 1 Keen in the upper right corner of the sky. You must hop onto the backs of the BLUE-WINGED mosquitoes to reach it. The black-winged mosquitoes will not support your weight, though.
Located past Bridge Bottoms, beyond the waterfall to the right.
The Exit is in the upper right corner of the area behind a door.
The key to the locked door is found on the left side of the area, in a wooden niche which is accessed by climbing down a pole which is guarded by a French fry-tossing Frenchy. There is a hidden Super Flower Power inside the underside of the key's platform.
The Grand Prize Area contains a Keen and a Cotton Candy. Get on the stone ledge that is to the left of the black hole in the wall where a little bird pops out occasionally. Jump up on top of the bird's head when it appears, then immediately jump up into the base of the wooden platform above. There you will find the goodies.
Another hidden Keen is found above the wooden platform just mentioned. From the platform, jump up left to the short sloping log. Then jump up and left into the base of the ground above to find the Keen.
Just north of Parsnip Pass is the weed infested Squash Swamp.
The Exit is due left on the other side from where you start.
There are 3 Boobus Bombs directly above the starting location. You must carefully traverse the floating lily pads to reach them.
There is a Super Flower Power next to the Exit, along a sloping bank of the swamp. Careful, don't fall in the water!
Squash Swamp is at a dead end in the trail in the Land of Tuberia, so when done, turn back south and east back toward Bridge Bottoms.
Brussel Sprout Bay
Northeast of Horseradish Hill, just around corner, is Brussel Sprout Bay.
There are lots of Candy Bars and Flower Powers high overhead at the start. You should hop across the lily pads to reach the loot.
There is a big stash of Candy Bars in the upper right corner of area.
The Exit is at the far right, bottom of the area.
West of Bridge Bottoms you will find the Rhubarb Rapids.
The Exit from this area is on the far left, bottom.
The Grand Prize Area is in the bottom chamber of the large wooden scaffolding structure found in the center of the area. A locked door inside the scaffolding blocks your access to the 3 Boobus Bombs, 2 Super Flower Powers and a key. So, you need another key first... Jump straight down off the left side of the scaffolding structure to the pegs sticking out below. A key is under one of the pegs. Using the [Down Arrow] + [Ctrl] key will let you drop down to the key location from the upper peg. Once you have the key, jump left across the waterfall onto the logs on the other side, and jump your way back up to the top, and then right across the top logs. Using a running jump, leap across to the scaffolding and climb down to your reward.
Since the Exit is also blocked by a locked door, you will use the key you find inside the scaffolding to get out.
Further west past Rhubarb Rapids, the trail leads to Grape Grove, a wide area with numerous platforms from which the Sour Grapes are suspended. Be careful while walking underneath them.
The Exit is at the far right, bottom of the area.
The main goal in this area is to get the 3 Boobus Bombs that are located in the upper rightmost corner of the area. They are reached by negotiating the many platforms and poles there.
There are also 2 Super Flower Powers in the area.
Grape Grove is located at a dead end on the trail in the Land of Tuberia, so after done, you should travel back east toward Bridge Bottoms.
Located west-northwest of Horseradish Hill, this is one of the more challenging and rewarding areas you will encounter.
The Exit is on the far left, about mid-way up vertically.
Watch out for the Pea Pods and Pea Brains. Though Pea Pods look mean, they can't hurt you... but their offspring, the Pea Brains which they toss at you, can!
To find the "cliff hanging" Keen: From the starting point, go to the left end of the trail, making your way up to the top of the dirt trail which you will find. Go down the long purple pole. In the chamber below you will find a Keen perched precariously above a steep drop off. With a little jumping finesse you will be able to leap up, grab the Keen and land safely on the ground you jumped from.
There are 3 Boobus Bombs in the upper left corner of the APPLE ACRES area, directly above the Exit location. They are easily reached by climbing the poles that lead to them.
The Grand Prize area is a little tricky to get to and to get out of... While standing by the Exit, walk right to the edge of the cliff; step off, then steer your fall back toward the wall, landing in a hidden cave. Watch out for the traveling melon car. Jump on it if you can, to be safe. At the left end of the hidden cave you will find 1 Super Flower Power, 1 Keen, and 3 Boobus Bombs. To get out of this area, take a couple steps away from the left wall and jump up, grabbing a hidden pole in the ceiling. When you reach the top of the hidden pole, jump up and right a couple times until you drop into another visible chamber with a pole that leads back to the surface.
South of Apple Acres is Spud City, home of the Tater Troopers.
The Exit is on the left side of the area, mid-way up.
There are 2 Cotton Candies high above the starting point. To get to them, walk left until you find a pole going up. Climb it all the way to the top of the wooden scaffolding. Jump off the right side of the scaffolding onto the supporting backs of the blue-winged mosquitoes. Keep jumping across the blue mosquitoes' backs until you reach the Cotton Candies. Remember: the black-winged mosquitoes will not support your weight.
There are 3 Boobus Bombs in the area: They are easily accessible by going all the way to the far left to the Exit. Jump up on the back of a blue winged mosquito near the exit, from there jump across to the pole and climb up. Then jump back to the right across 13 platforms until you reach the coveted Boobus Bombs.
The really tough Bonus Area: You can either leave Spud City at this point, or you can venture into the underground area where there are some very difficult-to-find objects... 3 Keens, 3 Cotton Candies, and 3 Boobus Bombs. Here's how to get started in the underground area. Climb down the purple pole that leads down into the ground. You will find a blue pole. You can climb all the way down the blue pole to the area below, and/or you can jump off of the blue pole about half way down it. Jump off to the left to gain access to the hidden regions to the left. The secret to finding your way around in the underground is to repeatedly test the ceiling above you by jumping up and hitting it with your (thankfully) helmeted head. There are many hidden openings which you can discover. Many of these openings lead into hidden labyrinths that contain the prizes mentioned. Good luck, and be patient. It's a lot like groping around in the dark. Note that in some places it is safe to jump into the water. For instance, if you spot the Eyeball in the water, it can be approached from its left by jumping off into the water and hopping across 3 hidden platforms in the water leading up to it.
This big castle is located in the northernmost, central place on the big map in the Land of Tuberia.
The Exit from the inside is in the top right corner of the castle. To get out of this level, you will need 2 keys to unlock the 2 doors which block the access to the Exit.
Note: You can jump up onto the stone gargoyle heads when their tongues are sticking out.
To find the 1st key: From the starting point go to the far right, climb up the pole which is there, go left a few steps to the other side of the gargoyle heads, and jump up and across on the 2 gargoyle heads to the next level above. There is a pole to the left. Climb up it to the next level. There are 2 poles to the right. (The farthest one will lead you up to a chamber where there is a free Keen. Get it!)
Exit the grape chamber to the left through the passage in the wall. Jump up on the wall top overlooking the deadly green flashing floor. Use a running jump to leap across the green floor to the platform where the key will be found.
To find the 2nd key: Return to the 2nd level from the bottom where the 2 gargoyle heads are. Go left until you are able to jump upwards onto several short ledges which lead up. There will be a Cotton Candy in the upper right corner of this locality. Jump the ledges all the way up to the top level hallway. Going right on the top level hallway will lead you to the doors blocking the Exit. Going left on the top level will lead you to a fire pit which you can jump over. On the other side of the fire pit is a window which you can jump out of. While falling down to the ground, you will grab a key suspended in the air, landing back at the start.
The Grand Prize Area: Jump up the stack of short logs next to the Exit onto the roof where you will find several Cotton Candies and a couple of Keens. The trick to getting the Keens is to jump up and grab the letter "I" in the word "ID" that hovers beside the Keens.
The Boobus Chamber
Here resides the great King Boobus Tuber. He is huge... seriously huge. And, he can jump as good or better than you can. So be careful!
Before entering here you should have collected 12 Boobus Bombs. You will need to hit King Boobus Tuber with all 12 Boobus Bombs in order to defeat him.
While throwing Boobus Bombs at the King, you must be constantly dodging him by jumping around on the platforms in the chamber. If you throw a bomb and miss him, you can pick it up and throw it again. Good luck! When you finally hit him with the 12th Boobus Bomb, he will self-destruct and you will have conquered your nightmarish adversary and win the game!