A martingale can be done with another piece of leather or usually with a brass chain. I'll attach a couple of photos.
The technique of creating a rolled or round strap is known in the saddlery trade as "round reins," which gets its name from its usage in horse reins. Some old school leathercrafters (like me) have a special bench mounted tool that is used to force folded bridle leather into a round shape. In doing so, the stitches are buried inside the leather. It is not trivial, nor cheap to make these round rein straps. But, to look at a perfectly round strap afterwards ..... ah.
rolled leather dog collar
