Turned edges with rounded corner

For something like the black flap, I was taught a technique when using leather exterior and fabric lining, but the concept is the same. For the wallet I made, I used kraft tex (a paper product but thicker and can be sewn, glued, etc.) cut to the finished size you will be wrapping the leather around, excluding the seam allowance. On the leather, you will cut little notches along the curve, making sure not to cut the notches too far in to the seam allowance or it will show. Apply glue to the leather all the way to the edges, and glue the kraft tex in place. Gently fold the leather over the edges of the kraft tex, excluding the corners, making sure not to bend the kraft tex, or it will mess up your nice straight edge. After all the edges are glued, come back to the corners and the notches will allow the leather to be smoothly formed around the curve. The notches will overlap, and it takes some practice to get it right. After everything is glued in place you can hammer the corners to reduce the thickness a bit. You would repeat with the lining leather piece, and then glue/sew the outside and lining pieces together.