This is the hardest step, so here's a step by step guide

1. try to join a game
2. the game won't let you join
3. cry

repeat these steps until you eventually join a game

After you get in a game and spend 8 hours trying to play as the monster, you might finally get it. Now select Boxy Boo and follow these steps.

1. Use the music to find a survivor
2. Attack them
3. Miss because your range is too short
4. Use your grapple
5. Your grapple pulls them 2 millimetres
6. Use your jump
7. Slam your face into a crash door
8. Somehow reach them and do damage
9. Repeat step 8 around 300 times and they will finally be out of health
10. Bring them to a trapdoor and then forget about them
11. They get saved

Repeat these steps 3 times for each survivor and then you will win the game