After obtaining 8 stars, you will be able to open the door in the back of the Rec Room. There is a picture of Mario in the room behind it. Going inside will get you into the stage.
There are 2 stars you can obtain in this level: one from a star switch, and one from finding the 8 red coins.
To reach Goomboss, you will need to reach the red X on the map.
Once you enter the level, slide down the slide, and you'll land on a checkerboard-like floor. Head to the left and jump over some tree stumps that move up and down. Don't go into the purple mist, it's poisonous and takes one hit point every few seconds.
Keep on the left until you meet a Piranha Plant. Kill it if you need to, then run past it onto the next tree stump, which has a pole coming out of it. Climb all the way up to the top of the pole, and a 1-Up will come out. Jump off, and head up the bridge. At the end, there are three stumps. One has another Piranha Plant, one has a Spinning Heart (run through it if you need to gain some health), and the final one has a hole in it. Jump down the hole to face Goomboss.
To defeat Goomboss, you will need to run around behind him, then eat a Goomba that is running behind him. Then, you can either spit it out or make an egg to hit Goomboss. It will take 3 hits to defeat him, but he gets faster with every hit. Upon defeating him, you will get the key to Mario's room, unlocking him.
Collect the 8 red coins. You need to have unlocked all characters and the ? Blocks to be able to do this.
- When you start, go down the same path as you would if you were going to fight Goomboss, but turn right just before the tree stump with the pole. You'll find a metal grille which you can hold on to. Make your way along the grille, collecting the coins as you go. The third coin is the first red coin.
- Head back the way you came and go up the pole. Instead of going on the bridge, make your way to the end of the lone branch. There is another red coin there.
- Slide back down the pole. Instead of going to the path with the Piranha Plant, take the other path, which goes to a tree stump that moves up and down. Get on that, and at the very top is the third coin.
- Go back to the Piranha Plant. Head right to the two moving platforms, going out to the lone stump. There, you will find a Black Brick Block, and a Goomba with Wario's cap (if you are not playing as Wario). Get the cap, and punch open the block, which contains another coin.
- Head all the way back over the three moving stumps. Go down the checkerboard path to the tree stump with a log in it, and turn left. In the middle is a platform which tilts depending on where you are, and in the middle of that is another red coin.
- Get hit by a Goomba if you came in as Mario, or grab Mario's cap nearby otherwise. Jump carefully on the tilting platforms to reach the stump with a ? Block. Hit it, grab the Power Flower, and quickly float up to the very top of the log to the right. The next red coin is on the top.
- Jump all the way down onto the side of the tree stump next to the rest of the course. Get hit by a Goomba if you came in as Luigi, or grab Luigi's cap nearby otherwise. Head back to the Power Flower, grab it, then run back to that side of the tree stump and backflip up to a hole in the stump covered by another grille. A red coin is there.
- Grab another Power Flower if you need to in order to get out. Head to the stump with the log in it, and go down the checkerboard path. Turn right and go down the pathway with the Brick Blocks and moving platforms, and there will be a Piranha Plant nearby. Past that, there are many moving stumps. Go to the last one, which has another Piranha Plant and the final red coin.
Go back to the stump right before all those moving stumps, and there's your star.
This star requires Mario.
Go down the path to right before the three moving stumps. Head down the checkerboard path, but turn right up the sloped path before the stump with the log. Be careful of the Piranha Plant, jump over the four tilting platforms, and you'll land on another path. Head down it to the switch, press it, and then run all the way to the end of the path and wall jump up to the top of the cage. At the top will be the star.