*First you have to create your desired shape (heart) with pipe cleaners.
(You can used colored pipe cleaners, or you can add a drop or two of food coloring to the borax if you only have white pipe cleaners.) I used white and pink pipe cleaners.
*Tie a string around it and hang it in a wide mouth glass jar with a pencil. Be sure that it will be free hanging and can be covered completely with the borax solution.
*Boil water. Add the borax to water:
3 T Borax for each 1 cup boiling water
*Stir until dissolved.
*Pour solution into jars until your pipe cleaner shapes are covered.
*Let sit overnight in a place that will be undisturbed.
This was just after a couple hours:
As you can see each one progresses differently. And a little side note, don't worry if your jars start crystallizing, just rinse them with HOT water when you are done and dissolve the crystals!
While we wait, here's an explanation as to why this all happens (courtesy wikipedia):
Borax is a material that grows into a crystal naturally. Just like snowflakes are crystals that form when warmer water in the clouds cools enough a borax solution will cool into crystals. The reason we use hot water to make borax crystals is because borax will disolve into water. Hot water can hold more borax than cold water can. So when the borax is stirred into the hot water until no more will fit, it is called supersaturated. As it cools, it cannot hold as much borax in each water molecule, so it realeases some of the borax into it's natural repeating mineral shape. It's just like a sugar crystal or a salt crystal. They will do the same thing as the borax when disolved in hot water, only they take a lot longer to crystalize.