I just sand it. I've made some holsters and some of the back was like that. I just sanded it until it was smooth and nice looking. Worked fine for me. I came out looking like a good back should.
Just punch 2 small holes in the front of the holster one about the other with about 3/8 ths between them. Then just thread your loop through them so the top loop and bottom ends are on the outside.
one above the other......sorry for the misstype.
This is my first leather work. I needed a knife sheath and thought I'd just try to make my own. Lots of research on the internet. Both In this forum and on You Tube. Turned out ok for the first time.