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.
Step 4: Prone to wiggle
There are two ways to go here, you can either extend your arms in front of your body or the safer way is to put your hands on the side of your body as though you were going to do a pushup.
My personal opinion is that it doesn't look quite as good to do the pushup style but WOW is it more comfortable. Bend your legs at the knee so that they are perpendicular to the floor. Then kick down and have your toes meet the floor.
Step 5: Activate... WORM!
If you opted for the arms extended position, now is the time to throw your arms above your head. Your toes, then legs, then chest and arms will come back to the floor, then kick yourself off again.
With your hands on your side, you can just land and gently place your upper body back on the ground. It's kind of like a rocking motion, kick off with your toes, roll your body up, then the toes connect with the floor again. Repeat.