Initial flake? That's the easy part.
First, if your knapping material is in cobble form you'll need to hammer off a spall using another stone. Once you have a spall (large flake) hammered off, or if you are starting out with a spall, use an antler billet (hammer) to knock off large flakes, working your piece down to the basic size and dimensions that you want.
If your spall is large, use a large antler for your initial work. As your spall reduces in size, go down to a smaller billet. I have one billet that is about a foot long and an inch across on the "business end" and another that is about six inches long and a half inch across on the end. The larger billet breaks off more material, thus reducing the size quickly, but it is also harder to control the flakes you break off. As you get closer to your finished size and dimensions, using the smaller billet will allow greater control of the flakes and will help you to break off the long, thin flakes that will make your knife beautiful.
Finish the edges by pressure flaking with the thin end of the antler, gently pressing off flakes where needed to sharpen the edges, to refine the shape of the knife, and to detail the back with notches and a stem if so desired.
What material are you going to use? How big of a knife are you looking at making?
coyo