Easy but time consuming. First I measure my guitar strap, it's around 60" and 3" wide. The leather store I like sells leather by the pound and I bring a tape measure to make sure I get the correct length or you can also make a cardboard pattern and lay that on potential strap pieces.
Sometimes you might have to buy a hide, you need two long pieces of leather for this project.
I like to cut the leather 4" and trim down the sides. This is done after all the studs are set and the sides are glued.