Any sharp, thin-bladed knife works, especially if it is not too wide, is smooth and slick. Single edge razors would rate pretty high, I use a Rapala fish fillet knife (largely because I HAVE it), honed sharper than a surgical instrument. Dipping it into warm water before trimming does not hurt.
Cut right down to just above the royal jelly. Your grafting will speed up quite a lot. Separate your QC bars from the QC frame. Graft one bar, insert it into the frame, drop the frame into the Cell Starter. Graft the next bar, pull the frame back out and add the 2nd bar, float it back into the Cell Starter. Graft the 3rd bar (if using a deep brood box), pull out the QC frame, add the 3rd bar, float it back into the Cell Starter right next to the pollen frame, and close up. A warm, damp cloth can be used to cover the grafts as you go, keeps them from drying out wile you are in the grafting tent or room or your pickup cab, or where ever you do your grafting.
Grafting off of black plastic foundation comb sure makes seeing the 80-hour old eggs-turned-larva (~8 hours as a larva) easier.