Before I chose the wood for the knifehandle I outlined the knifeblade on some paper. Then I began drawing the initial design for the knife I wished to end up with.
Play around with it a little, it will probably not end up exactly as planned, but then you have an idea as to what your goal is.
When pleased with the design, find the materials needed.
Tip: If in doubt on how the handle would fit in your hand, take some playdough and make a roll. Then take it in your hand an squeeze as if you held a knife, the size of the squeezed playdough shows the size your hand is most comfortable with.