Do I need a hammer or mallet to use these punches?

Yes. These are hand punches — you'll need a rubber mallet or small hammer to strike the handle and drive the tip through. A soft mallet works best to avoid damaging the handle.

Will these work on thick leather or just thin vinyl?

They punch through standard automotive leather and vinyl cleanly. Thick, multi-layer leather may require more force, but the hardened steel tip is designed to handle it without binding.

What's the best way to mark where to punch before I start?

Mark your hole locations with a fine pen or tape measure first. A light tap to start the punch, then a solid strike keeps the bit from walking across the material.