Spray Adhesive (you should probably use hot glue)
Iphone case 
Shelf liner segment 
Pen/paper/tape
x acto knife.

You should probably use a surface that you dont care about like a wood work table, First take the iphone case and trace it with a pen or what ever you would like. After your done tracing place a piece of the shelf liner over the outline and tape it down.

you do not want glue getting anywhere near your phone so tape anywhere near holes on the case... Make sure the phone is out of the case before you put on the adhesive. you should probably use hot glue around the edges to limit the clouding of the glue.

All you really have to do here is trim the excess if there is any and now you have a new iPhone cover!