Nemia
Work slowly under good light from the left (if you're righthanded). Use good tools, heavy scissors, good corner rounder. Keep your elbows on the table to get maximum control over your hands. Move the card not the tool - let the scissor cut straight and adjust the card to the scissors, not the other way around. And if it doesn't turn out perfectly done - it doesn't matter. Only you will notice, and it's a sign that it was done by hand and not machine. You're not a machine. Give it your best effort and don't be a perfectionist. You're personalizing this deck so it will carry the traces of YOU. No need to be perfect, just do you best.