The main reason I chose to make my own folder is because most of the nicer ones you find on the market nowadays are over 3" long or are a bit too wide to fit comfortably in "girl pockets" (if you've never experienced them you have no idea how lucky you are). Plus I figured it'd be a nice challenge.
**Note** Please make an effort to read the annotations, I think they're more helpful than the formal instructions.
Step 1: The Design
Remember that you need to leave room for standoffs or a backspacer while the blade is in the closed position (see last pic). Also you should put a fair bit of care into figuring out the placement of the pivot and adjusting the blade length accordingly. If the pivot isn't aligned properly you're liable to end up losing a lot of blade and handle width by trying to correct it (or you could turn the misalignment into a "feature").