Here's how you do it:
1. Start with a square piece of paper.
2. Fold the paper in half, then fold it in half again.
6. Use your finger to make an opening, then flatten the opening to form a triangle. Repeat this on the other side as well so it's symmetrical.
10. Fold the rectanglular paper in so that it forms a square behind the heart, then fold those squares into triangles so that they are hidden behind the heart.
You can make these little guys in any color or size, and if you don't feel like using them as bookmarks, you can put them together to make a heart-flower:
Or you can just leave a pile of little origami hearts on your table like this:
Any way you fold it, it's going to be cute.