Download a copy of Bob Turzuola's book and read it cover to cover. The first thing you need to do is have a good grasp of how all the geometrys interplay with each other. Without that the rest is just playing at drawing. If you are designing a flipper then that adds a whole other dimension of considerations but a standard stud or Spydie hole or whatever is much simpler. Decide how you want the knife to stop in the open postition, whether it be internal (probably not necessary), standard stop pin or stud/stop. Make copies of your drawing and cut out the blade leaving plenty of room to trim the tang area later. Poke a pin through the pivot while the blade overlays the copy of the entire knife. Pivot should be half way from top to bottom but the distance from the front needs testing on your end to be just right. Open and close and adjust the pin a bunch of times. Make sure you allow room for the handle to protect the tip when closed. Allow room for your backspacer or standoffs.
I like to Scotch tape the paper at the pivot because of all the holes. If you think you have it right then move to a solid mock up. I like 1/4" wood for the handles and aluminum sheet for the blade but just about anything ridgid will work.
Considerations: lockbar length, lock location, detent path, stop pin. RJ Martin told me a couple of years ago that he aims for an 80% blade to handle ratio and I think that's a good goal (eg. 4" blade to a 5" handle).
There is soooo much more to it but that should get you started. Seeing how one adjustment always seems to screw everything else up is half the fun. It's very rewarding to get the whole thing to work right, especially a flipper, but after that you realize that was all the easy part.
