Really, its all about waiting for our opponent to get into the outermost limits of Marth's range. By "outermost limits" I don't mean tipper range. I mean the farthest out Marth can go while still being able to recover to the stage. Its all about jumping off the stage, tipping a fair, using your double jump, and doing DS back up to the stage. Of course, thats not all I do to gimp. There's the ledge dropped bairs and such, but going out far with fair is my main gimping tool.
Your main issue with gimping is definately going to be surviving. At first, you'll probably going to kill yourself ALOT. I know I did. Every other time I went out to gimp, I'd end up losing a stock. Learning the boundaries of Marth's recovery will be a problem, but that fixes itself over time. The more time you spend off the stage, the better feel you'll get for surviving.