Please note: I'm about 5-7 years past my prime for doing the worm. As you read through this, take pity on an old white dude who thinks he can still (if he ever could) dance.
Step 1: Video. Fresh.
Thank you to Instructables for suggesting a much clearer video.
Step 2: Prep Steps
Step 3: Entering the worm
Since this was originally posted, I've added a new video which more clearly illustrates this.