I've added leather coat on my phone's case from an old purse.
Step 1: Find a Piece of Leather and Make a Template.
In my case there was a metal coat on the original case that felt off so I just replaced it with leather instead.
Step 2: Cut Until It Fits.
Make sure the cut is precise. If there will be leftovers it will peel off over time. Use a very sharp knife. After you finish cutting, burn the edges a little bit so it will be smooth.
Step 3: Glue Everything in Place and Let It Dry.
Make sure that there are no wrinkles and let it dry.
Step 4: Enjoy Your Homemade Awesome Case!
I thought about adding stitches on the edge for decoration but haven't done it yet. Feel free to be creative!