There are many different techniques different knifemakers and bladesmiths use to design their knives, and methods vary from maker to maker. That said, this is not the only way, the best way, nor the most commonly used way to design a knife, it’s just my way.
I start out two ways, one by making a blade and designing the handle off of that, or designing the whole thing prior and then working off of that. Unless if I’m doing a custom job or stock removal, I tend to do the former way.
In the former method, I start out imagining what sort of blade I want (skinning knife, bowie knife, dagger, etc.), taking bar stock (rough untouched steel), and begin forging it to the general shape I have in mind, tweaking and refining as I go along to whatever is aesthetically best. Once the blade is done or near done and shaped to how I want it, I trace it on paper and work the handle off of it.
If I am to design the knife before making it, I sit down with a pencil and paper and the type of knife in mind, and always start out with sketching the blade. I make two dots on the paper, to mark the tip of the blade on one point, and the other end of the blade at the other, based off of what length I want. I then slowly sketch in between the two points, making many small strokes, until I have the general shape of a blade in mind. I then erase these strokes and do it again, (erasers never completely obliterate the trace of pencil marks) erasing and re-drawing until I get a shape I am happy with. Once I’ve done this, I use an interesting trick I learned a while back. I draw a “flow line”, from the tip of the blade and through its center, following the “flow” of the blade. I continue this line in the same flowy motion past the blade and into the blank space behind. I lightly erase this line and begin sketching the handle, using the flow line as the central skeleton. Using this technique gives a very good flow from blade to handle, which is essential to a good design. As a solid rule, if the knife has a bowie-style ricasso, I always draw the beginning of the the handle the same width as the ricasso, which further adds to the flow. I sketch, erase, and repeat until I get something I’m happy with, copying different knife styles especially with the pommel. I then draw in the guard, copying whichever type I like best. I never draw the front portion of the guard further out than the heel of the blade.