I have made a few molded knife cases for different shaped knives. I have experimented with a few different ways of forming them and I personally like to make a "mold set". What I do is I get a piece of either 3/8 plywood, or 1/2 or 3/4 craftwood (which is uaually pine, from Lowe's) that is big enough to clamp my mold to, and use this as my base. For most of my molds, this is about 3"x6". I then get a piece of plywood the same size, and draw the outline of the knife that the case will be for, making sure I leave enough room for the thickness of the leather around the knife. I have also used Plexiglas for the base and the form (the outside part)
I cut this out and wax (if it's wood) the form so it won't hold the moisture from the molding process. Make sure the knife will be able to be removed from the completed case. If I want to use the knife as my actual mold, I make sure it is wrapped well in cellophane to protect it. If I want to use a wood plug, I make it the same thickness as the thickest part of the knife, then cut or carve the recesses and curves on the part that will be the top when molding. Again, make sure this plug will be such that the knife can be removed from the completed case. Then I also wax it to prevent molding. Then I screw it to the base from the back side, so the screws wpn't leave marks in the case.
I cut my leather oversized, of course and soak it well, usually 1/2 hr or so in warm water, till it is completely limp. Then I use a wood roller to squegee some of the water out, so it will set faster. Put it on the form of choice, and using "C" clamps draw the outside form down over the knife or plug. This makes the leather stretch to the knife or plug and take it's shape. I make sure the clamps are tight and leave the setup over night to "cure" If you want to do a little carving, let it set till the moisture content is right, then carve while it is still clamped, but be careful as the leather is bieng stretched, and will have a tendancy to spread the cuts! After the leather is dry, remove the form and continue with the assembly process.
I hope this makes it as clear as mud. I know I can't explain exactly what I'm thinking, but I do the best I can.