I've made a few assorted belts in my time; never a dirk belt as it happens. Like Tobus, I'd recommend a half hide over a double shoulder -- you want that kind of length which you can cut, and you want to cut belts only from the uppermost half of any half hide anyway -- this is where the leather is densest and strongest. A belt from there has a modicum of stiffness to it that is handy in belts -- the soft floppy leather from down in the belly regions is suited better to soft-feel things like pouches, sporrans, wallets -- anything that doesn't need to be at all stiff but is better if really flexible. Nice soft cuarans, say -- and their thongs or laces. There's a leather tool specifically for cutting leather lace from bits of otherwise unusable leather scrap.

Double shoulders are for things that don't need to be as long as a belt -- or else you're going to end up piecing your belt together by skiving, gluing, and sewing pieces together. But you can do better.

A half hide, dividing a full hide lengthwise down the spine in a long clean cut as it does, makes a superb starter, with that StrapCutter, for a good stout belt of simply any likely length, and sporran straps too. It's more expensive, but it gives you a leather supply for many years for a remarkable lot of stuff.

Leathercrafting's a fun craft too. It even smells good. The tooling and carving give artistic expression even to somebody who's in a lifetime habit of downplaying his own creativity (phooey to that). Though the dyeing will stain your fingers something awful -- likewise anything permeable your wet dye-covered fingers might happen to touch. It doesn't so much wash off as wear off -- it is intended, after all, to color dead epidermal cells, and that is exactly what's on your skin.

Finish any belt's edges with an edge slicker or else a bone folder. Teamed with an edge beveler tool, which knocks the corners of the leather off to then be rounded over and glazed with the slicker or folder, this finishes the otherwise raw-looking edges of the belt and completes the look of the thing.