The gusset and welt are made of one piece I use a 3" strip of leather for it. It sewn from what will be the inside of the sporran.

If you attach the gusset to the front face I use contact glue and bull dog clips to clamp it in place

sew from the bottom up to one side then sew the other.

Here's a quick pic, the brown leather is the face and its clamped in place for sewing