Nose the bottom blade of some scissors under the ziptie, so the scissors are parallel to the wires. Grab the wires/cables with your other hand and bend them away so that on top, you just have scissors/ziptie/air and there's nothing else the scissors could cut. That's the way that's worked for me, ever since I accidentally cut through a tiny fan wire on my first build and stupidly tried to reattach it, and nearly started a fire.