Two ways to do this:
You need a strip of paper no wider than the mat overlap. Fold it once to make a 'L' with the corner clipped. Position that with the limbs pointing away from the print and tape the limbs down with a diagonal strip of archival tape. Strips cut from properly processed scrap prints work well. I have also seen people put in two creases at 45 degrees to mark the corner, and tape the strip down diagonally to the print corner. Bigger prints benefit from the pocket approach.
To do it with a single piece of tape means making a second fold to take one limb in the other direction to get the adhesive faces on the same side. It works with gummed or removable backing tape. I don't like it because the resulting corner is three layers thick, but it does make a pocket for the print corner where it cannot com into contact with the tape adhesive. You also need to print with enough border to allow for the tape width. Nice to know how to do it, though.
I confess that for around 8x10 matted to 11x14 I often print on an 11x14 sheet with a 4-blade easel and just sandwich the backing, print, and mat together with a linen tape hinge. Self-adhesive archival corners are available, too.