Using a sharp knife, cut a hole in the top of your pumpkin that is almost big enough to fit your plant in, container and all. I eyeball the size, and cut the hole smaller than I need. Once I get the initial hole done, I hold the pot over it and get a better sense of how much bigger to make it.
You can also break or cut off the pumpkin stem (though this isn't easy) and use the bottom of the pot to draw a circle, marking the area to cut.
