I make sporrans, not too hard to do, a little time consuming.
The one you have pictured. the back and the flap are one piece. made from 6-8oz leather and lined with garment leather or 1.5-2 oz leather. You can glue the lining leather to the heavier.
They are about 7" wide and 8-9" high. The front piece is identical to the back minus the flap and an inch or so for the bend.
Sew a gusset 2-3" wide to join the two pieces. Make tassels.
Basically the same construction as a ladies purse.
You will also need a belt loop on the back for a sporran strap