Fold a square piece of paper diagonally twice.
Fold three corners into the center, but leave the fourth extended.
Fold the bottom left corner up to the upper right corner.
Fold the top left corner down to the bottom.
Finally, fold the corner which has been left out to the side into the opening of the top flap. You now have a triangular bookmark to put over the corner of the page you're reading.
Here's the second version:
Fold a square piece of paper diagonally.
Fold the bottom left point up to the top.
Fold the bottom right point up to the top.
Unfold the two points, and fold the middle point straight down so it touches the bottom.
Bring the bottom left point up and fold it behind the top layer.
Bring the bottom right point up and fold it behind as well. Now your bookmark is finished.
Here are the two completed bookmarks.
Here it is placed over the page.
Happy folding! Better yet, happy reading! May your place never be lost.