Depending on what you’re wanting to make it doesn’t take many specialized tools. And you could probably get away with things you already have. A straight edge, exacto knife, if you have a pair of calipers they’re great for marking stitch line from edge, ruler for marking spacing and a stitching awl will go a long ways. The stitching awl can be made from a nail driven into a block of wood for a handle and ground into shape then sharpened. For needles use saddle needles that are blunt so they’re easier to guide through the punched holes. The more specialized tools make it easier, faster and more reproducible results. But for a couple one offs that’s no big deal. And before you mark the leather, make a paper template to make sure it’ll fit what you want before going to the leather. And a pencil works fine as the graphite can be wiped off and smudged covered over with stain if you stain the piece.
I hope that helps some.
Kyle