The first thing to do is to determine how long your strap has to be from button to button. If your customer has an old one you can measure, you're ahead of the game. If not, get him, his guitar, and yourself together with a piece of twine, rope, an old belt, or a piece of scrap, and see where it hangs to be comfortable for your customer. If your design is to be one piece, you're off to a flying start and just have to figure how to decorate it, if at all. You need to leave either 3/4 or a full inch of leather beyond the button hole and you might want to think about reinforcing the hole, depending on the weight of your leather. If you're going to put a buckle on it, find out before you cut anything just how wide the buckle is going to be. Then you can allow for turn-back to hold the buckle (and keeper?) and how much of an over-lap for the tongue. Keep in mind that you don't have to make it the same width for the full length: you can taper the ends like on a belt. Some of my most popular straps have been two or two and a half inches wide tapered to a real fancy buckle set. Engraved silver (plate or over-lay or solid) really sparkles under stage lights. Heel bar or center bar will work. Don't even consider a plaque type (trophy) buckle. They have a tendency to come un-hooked during performance. I hope this helps. Have fun. JimB