Instructions
Collect the Necessary Items
Click anywhere on the screen to wake up Guybrush after the Voodoo Spell cut-scene. Go up on deck. Another cut-scene will occur in which Guybrush discovers he has arrived at Monkey Island.
Go to the Storage Room. It is accessed via the hatch on the deck. After going through the hatch, go through the second hatch you see. In the Storage Room, pick up the "giant piece of rope" and open one of the kegs on the left to get some gunpowder.
Return through the second hatch and enter the door you see in this room. You will now be in the kitchen. There is a pot on the table. Pick it up. Return to the ship's deck.
Prepare the Cannon
Use the gunpowder on the cannon's nozzle. Guybrush will arm the cannon with the gunpowder.
Use the "giant piece of rope" on the back end of the cannon. Guybrush will put the rope in the cannon's fuse chamber.
Return to the kitchen. If you have made it to this point of the game, you will have at least one of several T-shirts that are given to Guybrush at various points. Use whichever T-shirt is in Guybrush's inventory on the flame beneath the Voodoo Cauldron. It will become a "flaming mass."
Get in the Cannon
Return to the deck. Get close to the cannon and use the "flaming mass" on the "giant piece of rope." You will only have a few seconds before the cannon fires.
Use the pot on Guybrush. If you did this fast enough, he will put the pot on his head and climb into the cannon, which will fire him off to Monkey Island.
Return to the Storage Room to get more gunpowder if the cannon goes off before Guybrush can climb into it. Repeat the first two steps in this section until successful.