Skiving ("sky-ving", to skive) is the process of thinning a piece of leather. A lot of the leather available isn't thin enough for Stablemate scale tack, and in some cases isn't thin enough for Traditional scale tack, either.
Even if your leather is the right thickness, you'll still want to skive the areas where pieces join so that they flow smoothly into each other. It's recommended to skive where you'll place buckles, too, so that you can reduce bulk.
It's a pretty essential skill, but if you've never done it before it can be confusing to figure out! I use two primary methods: a snap-blade knife and my Dremel. Both methods are pretty simple but hard to describe. I decided to show you instead!
Skiving: Snap-blade Knife
A quick note here - this method of skiving should be done on a smooth, solid surface like a glass cutting mat (in the video) or a piece of marble. A marble tile or strip from a home improvement store is good enough!
As a side note on the dremel method - cover your workspace, be aware of any delicate machinery (sewing machines, computers, etc) and definitely consider wearing a mask over your mouth and nose and eye protection! The dust produced is extremely fine and tends to stick to itself, but it can wreak havoc on your respiratory system, especially if you're asthmatic (ask me how I know that last one...).