Tim,
I am not sure I have ever worked with pigskin outside of some gloves I repaired, so please keep this in mind. Chuck may have a better way of doing it, but here's how I do it.
What I do when wrapping leather around an edge is to use a compass or some marking device to lay out the line for stitching around the outer edge of the top of the flap. Then mark the holes with wheel and punch the holes with an awl while trying to keep the holes as perpendicular to the leather as possible. This also gives you a distance you can measure and cut the width of the piece of leather you will wrap around the edge - just make sure you measure around the edge from the top of the hole to the bottom AND add enough for an edge of leather beyond the stitches on BOTH edges. (Yeah, I forgot to add that extra distance a time or two I must sheepishly admit.) For example, let's say the distance from the top of the hole around to the bottom of the hole is 1/2". You should add at least 1/8" on both sides or maybe even 3/16". That would give you a total width measurement of the piece that wraps around the flap from 3/4" to 7/8". Also when you are first doing it, it is not a bad idea to cut a couple short pieces of leather and punch the holes and then see how it will wrap around the edge when sewn. Trust me, that saved me a couple of times.
Now that you have the width measurement, it is time to work the leather you will wrap around the edge. Pigskin can be really "stetchy" and to a lesser degree - so are other thin leathers or splits. I "pre-stretch" the leather by pulling it both length and width wise BEFORE I mark the holes and cut out the piece that wraps around the flap. I try to get all the "stretch" out that I can. I try to get as much of the stretch out as possible. Then and only then do I use a long aluminum ruler to lay out the cutting lines to get the width I want. I also use the rule to lay out a line for the wheel and mark the holes before I cut it out. That will give you a more uniform edge beyond the stitching holes. After you cut the piece out and punch your holes, CHOOSE the side of the piece that is more uniform and put that on top of the flap where it will show.
Now, there is another way to do it. You can do everything the same up to this point, but ONLY punch the holes on one side of the binding strip. You put that edge on top of the flat. You punch your awl through the holes in the edge of the binding strip and through the flap pieces and make a fresh hole on the bottom edge of the binding strap where it "naturally" comes through. You only punch one or two holes all the way through both flaps and edge binding and sew it up before you punch more. That gives you a little leeway for really stretchy leathers and only the bottom side will look not as even. I don't like that way as much, but I've seen some people use it very successfully.
When I sew on that binding strip, I dampen the leather so it will work around the curved sections of the flap/s easier and better form to the edge. I would suggest that either way you do it. The leather will dry as you sew it, so keep a damp sponge handy to dampen the leather as you go along.
I'm not sure if this any clearer than mud, but let me know if you have questions.
Gus
