How to Start Writing Scripts

As a junior with very little script writing experience, I found it challenging to get into the spot. I'd have an idea for a spot. But getting to that idea always seemed clunky.
Recently, I was invited to speak to a class at the Temerlin Advertising Institute at SMU, and we talked about writing scripts for radio and TV. Here's an exercise we did together.
First, watch this classic SNL clip.

Okay. Now, take out a piece of paper and write the first sixty seconds of this skit. It's not a memory game; you don't need to remember the kids names. But how did the skit begin? Go ahead, try it.
When you come up with a great idea for a TV spot, it's a lot like saying, "What if Chris Farley were a crazy motivational speaker who really lives in a van down by the river?" It's a funny concept. But that's not the first line of the script. You have to begin with "Open on a living room." And you have to write some dialog that's not all that funny, or even memorable. But it gets you to the funny and memorable part.
As you watch TV - sitcoms, dramas, commercials - pay attention to how they begin. What are the first lines spoken? What is the first image you see? Figure out how how those elements serve as a base, and how they lead to the parts you really remember.