These photo corners make mounting the print onto the backing board very easy.
I tend to use the .625" corners so that there is no risk of the photo corner peeking out from behind the mat, but if you have a generous edge on the paper, the bigger sizes are much better.
Search online for these descriptions:
Lineco Infinity 0.625" Acid-Free Archival Photo Corners
Polypropylene Mounting Framing Corners, Slimline