If you need to make a hole under the spike in the strap, then you can do it in 2 ways:
A) using a special tool - punch.
B) with the help of improvised means
The punch is a metal tube. You put it on the strap, from above, hit it with a hammer, sharp edges of the tube cut in the skin hole (round or oval - depending on the type of punch). Under the strap it is desirable to put a piece of thick skin, so that the tube is not dulled from the impact on the floor, for example. The hole is perfectly flat.
If there is no punch, then we use an awl, sharp narrow knife or razor. Shilom planned the place of the future hole, pierced the skin. With the tip of the knife, in a circular motion, we remove the leather shavings in a circle, widening the hole more and more. Actions alternately produced on both sides of the strap - both from the outside, and from the inside. Here is an example of an elongated hole. If you do not hurry (and the process takes about 30 minutes), it can get very accurate.