Step 1: Watch the Video!
Step 2: Forming the Gas
Step 3: Ignition
I found it actually ignites most consistently with a hole just small enough to poke the fuse through, and works better at the back of the pumpkin. I only placed it at the top here so it would be visible.
In reality, placing the fuse near the opening at the top leads to pre-mature ignition more often than not, and isn't recommended.