Super Nes repro cart sticker, best way to cut out? merci

[emspea]

Hi all.

Im having trouble with the snes stickers i make for my carts, I have them printed on high glossy adhesive paper.
That part is ok, I then apply clear tape over them one by one and get the air out.

When I go to cut with a stanley knife, I always cut some of the sticker wonky. It's really annoying!!!!
Is there a way to do this which will get me straight lines everytime? Im using a ruler to guide my knife, but my grip always slips a little.

Also is there any other method for applying a shiny coat to the sticker other than clear tape?

Many thanks, kind regards.

merci

Pea.