It really depends on what your horn has for attachment points. Do you have a horn yet?
Many earlt horns have a staple: the leather will be fed through the staple, folded over and sewn to itself. Some horns have a staple at the front as well.
Others have a draw pull looking end - in this case, the end of the strap has a button hole in it and it slides over the end.
For the front of the horn, If you dont have a front attachment staple, there are two ways they would attach the front - most horns have flat or narrow spot near the spout, where you can attach a strap and it wont fall off.
1. You can wrap the leather around the spout and sew it tightly.
2. You make a loop at the end of the strap and then use leather chord to tie around the spout to hold the horn to the strap.
As far as buckles go, many f&i were set up without buckles, but a narrow buckle (½ to ¾" wide) could work
Ive attached a few horns that use or are inspired by and their strap attachments