Guitar straps ship with holes cut for a normal strap button. But if one of your strap buttons is also a 1/4" plug then you might want to enlarge the hole on that side of your strap.

You can buy a hollow leather punch set for very little money on Amazon. Based on some quick eye-balling I think about 3/8" is the right size.

Add a hammer and a plank of wood to put underneath the strap and you're ready for action.

I find that it works better to cut into the leather if I tilt the punch around rather than hammering it in squarely.