I use a yard tool from big squeegee (I am starting to sound like a salesman).
1. I usually print/cut to 1/8" smaller than the size for magnets and real estate signs with slightly rounded corners.
2. Place on mag and align using two corners making sure that the space around outside is equal.
3. While holding in place put two pieces of masking tape, maybe 2" long. One on top center anchoring print and mag to the glass table, and one on the bottom center doing the same.
4. Fold over first 1/2 onto itself then peel back liner and cut straight with scissors.
5. While holding print up a bit by the very edge slide yard tool underneath and push it all the way to the end. The mounting part only takes maybe 1 second per side and it comes out perfect every time.
6. Remove 2 pcs tape and mount the other side without removing backing paper, it slides forward on it's own when pushing the tool.
This is a really fast way to mount, you can re-use the tape pieces and mount 100 real estate signs in no time. One of my favorite tools that I wouldn't be without. Also you can train someone to do this in less than 5 min and they don't have to ruin any.