After finishing the recipe, open the inventory and hold the down arrow key to scroll down while walking to the hatch. Then, after picking up the gunpowder and the rope, walk to the ladder. While Guybrush is walking, open the inventory, press the down arrow key and then the left arrow key so the gunpowder gets the yellow box. At the same time, move the cursor to the top right corner of the screen, making sure the cursor doesn't enter then inventory during the process.

This way you'll be setup to quickly use the cannon to finally get to Monkey Island.