If you can, borrow one of similar style that you'd like to make. Trace it onto some paper for a template. Add a little extra for sewing, and be sure to accommodate where the cantle will mount. The front and back should be basically the same. If you plan to use a gusset, that should be easy enough to figure out.

I've heard that it's easier to sew the sporran tab on the back and whatever closure method you use prior to sewing the sporran pieces together.

Correct me if I'm wrong folks, but I believe common practice is to make the body out of leather first, then attach the fur, prior to sewing the pieces together.