Step 1:
The other side of this paper is white,
prefold over these 4 lines.

Step 2:
First valley-fold over the 2 lines at the bottom right.
Then mountain-fold over the 2 diagonal lines in the
middle. And valley-fold over the 2 lines at the top left.

Step 3:
Valley-fold the flap upwards, do the same on the
backside. Then rotate the model, see arrow.

Step 4:
Open the model, see arrow.

Step 5:
Valley-fold over this line.

Step 6:
Make an inside reverse fold over these 3 lines.

Step 7:
Close the model again, see arrow.

Step 8:
Repeat steps 4 to 7 on the left, mirrorwise.

Step 9:
Mountain-fold over these 2 lines.

Done, real handy!

This is an Origami Video with instructions for the Heart shaped Bookmark: