1. When tacking the tissue to the print, as Greg Davis wrote above, tack only one spot in the center. Do not drag the tacking iron across the print--and certainly not in an "x."
2. When tacking the trimmed print and tissue to the mount board, tack two adjacent corners ONLY. If you tack all four and the print is not PERFECTLY taut, you will get a wrinkled print.
3. As Greg also wrote: Cut your overmat first. Then position the print in the overmat. No straight edge needed. No measuring needed.
4. When lining up the overmat with the mount board, make sure the top is aligned--not opposite corners, not the bottom. usually one hinges the overmat to the mount board, and since you will be hinging them together at the top, it is crucial that the tops are aligned.
There are probably a few other things, too, but these are the ones that come readily to mind.
Michael A. Smith