Collect two 2 liter bottles from anywhere and wash them out with soap and water. Also, remove any labels on the bottle.
Cut the two in a diagonal direction. The best place to start is where the label would have been and cutting down from there.
Seeing as the edges would be jagged from the knife, take your scissors and cut off any access plastic until you are satisfied with how it looks.
Paint the outer layer. (I usually do three layers.) The best way to do this without it being too messy is to put it on your hand.
(Optional) Apply paint to your canvas that resembles blood.
After your paint is done drying, take out your handy Hot Glue Gun and glue it down to the canvas so that they are close together. You may want to glue a little portion of the front together so it stays shut.
Voila! Your 2 liter bottle heart is now created. Enjoy it or give it to someone for a unique Valentine's Day present. (: