Use scrap leather if you have, you don't need a large piece.
Cut it in two even rectangular-shaped pieces. I used my iPhone to measure precisely the width and hight.
This is optional. I used soft fabric to line the inside of the leather, allowing for a smooth surface for my iPhone to lay on. I cut the fabric the same dimensions as the leather scraps.
Using glue gun, glue the fabric to the leather. Make sure not to get any glue on the visible part of the leather, it won't look too nice otherwise .
Once you've finished gluing the fabric to the leather, use a sharp knife and cut the excess fabric that may stick out. Giving the leather a nice, clean look.
Then take a sewing needle, preferably a strong one, and make holes in the leather to mark where you'll be sewing the two leather pieces together to form the case.
This is what it should look like. Then align the two pieces on top of one another.
Using thick thread, sew the two leather pieces together. You may use any color you like.
Use the sharp knife to shape the top flap however you like. Then make a small incision in the middle of the flap.
Using left over leather scraps, cut a long, thin leather piece making a string. Then thread it through the newly-made hole.
Tie a knot with the leather string on the top flap. Make another hole in the actual case and using more leather string, tie a knot once more.
Make a loop using the remainder leather string and glue it carefully in place.
Bam! A professional-looking iPhone case made of genuine leather. You can also carry cash in it, making it a little iPhone wallet!