My Chihuahua has always had a sensitive part of her neck where the buckle of her collar rubs against her fur causing hair loss and irritation. After trying out several collars with buckles I came up with a idea for a small leather piece that I call her collar keeper that fits just behind the buckle providing a bit of protection from the friction of rubbing.
Step 1: Before You Start
The first thing you would want to do is get a thin flexible scrap of leather. Tandy Leather Company or Hobby Lobby sells huge bags of them or you can buy trim pieces at a Michael's, Joann's or local craft supply store. The one i used is 1.16th inches and is possibly deer skin. You will not want to get vegetable tan considering its thicker and not "finished". Look at the thickness of the strap of your collar and try to find a piece that is slightly thinner than that. Note larger collars will require thicker leather as it will wear more on larger or more active dogs.
Draft a pattern roughly the size of the buckle. You can trace over it. I like to make it smaller by a few millimeters. It should be large enough to provide at least an 1/8 in. on the top and bottom of the collar strap and should ideally be slightly larger than the entire buckle to ensure it adequately protects your dog's neck. With a pencil or other marking/scribing tool score the two slots where the nylon will go through on the leather on each side of the leather.
With a punching tool or punching stamps Make two holes on each end of the scores. If using a punch, punch suede (rough) side down. You can skip this if you don't have the tools I just get cleaner cuts this way. If you are using a punch and are not getting clean holes check the inside of the punch dye for leftover leather pieces.
Trim the excess square bit between the holes creating a long slot. String one end of the collar strap though the leather smooth side out just after the d ring but before the buckle. Loop the other end and string it through the other side. Trim as you feel needed or to get a better fit. as you can see in my pictures you can see you can barely see it behind the buckle.