I'm working on a similar, first-time project and found it very helpful to look at the photos of sporrans on vendor websites. Scotweb in particular has photos from several angles that helped me suss out some details (that I couldn't figure out from the other sporrans I have on hand, a luxury you may not enjoy).

At the rear, generally the gusset is sewn wrong side of gusset to front side of back panel. This leaves two raw edges revealed. For plain leather, it seems easiest to burnish these edges and leave them showing. I am working with fur, including fur gusset, so I plan to sew a thin strip of leather over those raw edges, all the way around the outside of back panel.

As for lining, the plain leather sporrans I have/have seen are generally not lined.

Pictures are worth 1K words, I strongly encourage you to surf the commercial sites as well as some of the other DIY sporran threads here on XMarks.