Here are a few examples of some of the different places you will find slots on your model tack: boots with buckle straps,stirrup leather keepers, and d-rings on the front of a saddle.
When I first started making tack, I'd simply make a small cut in the leather with my X-acto knife. Unfortunately, I didn't have a lot of luck with that method. More often than not, the slot ended up being much too long or not quite at the angle I'd planned. Eventually I smartened up and figured out a way to make more accurate and consistent slots. Here are some of tools I use: a cutting board, a mechanical pencil hole punch and mallet or an awl, an X-acto knife with a sharp number eleven blade, and Gum Tragacanth.
The first step is to lightly mark the planned slot location on your leather. Using either the hole punch or the awl, make two small holes on either end of the slot.
The next step is to "connect the dots" with the X-acto knife. You can do this with one cut, but I prefer to make two. The advantage of the two cut method is that rather than just slitting the leather, you will actually have an open slot to thread your strap through.
Use the awl to remove the little piece of leather between the holes. At this point, you should have a usable slot. However, I like to go one step further and slick the slot's inside edges. I do this by dipping my awl's point into the Gum Tragacanth and running that along the inside of the slot.