We made page corners in public school and I used them at University to mark pages that would be important to me in writing the paper I was working on. In thinking about a quick valentines craft for our granddaughters to do these corners came to mind.
Step #2 Fold each side point to the top point.
Step #3 Open the side folds back up and fold down one layer of the triangle so that the point meets the bottom of triangle at the halfway mark
Step #4 Fold the side pieces over the single fold down and tuck them over the flap and tuck them inside.
You have now made a page corner and all you have to do is decorate them.