1. Use a flat thin board, about 6 mm MDF. Place the dummy phone on it. Place wet leather on top. Use lolly sticks (Popsicle sticks) on top of the wet leather and then use bulldog paper clips to hold in place. You may have to go around a couple of time to get the leather even and the sticks & bulldogs tight to the shape. Don't skimp on the leather
2. Use two bits of wet leather. One piece on each side. As above, sticks & bulldogs. And as above you'll need to go around it a couple of time adjusting it all
This is pen case done in that manner;