First, shape pipe cleaners into any old shape and tie them to a pencil. Mix a ratio of 1 cup of boiling water : 3 T of Borax and pour them into canning jars and suspend the pipe cleaners in the Borax/water mix overnight.
Why does this work? Because hot water molecules move farther apart from one another than cold ones. When the water is hot, the dissolved Borax has places to go between the water molecules. But as the water molecules cool down and move closer together the Borax gets pushed out of the way.
All Borax crystals, no matter what size, have the same shape. Long rectangular shapes with flat sides. As the Borax gets crowded out by the water, they begin stacking up on one another (like Legos) onto the pipe cleaners. If the water is super saturated with Borax the pipe cleaners will only be able to hold so much and some Borax will crystalize at the bottom of your jar.
There you go. Now you can make fancy shmancy Borax crystals but more importantly now you know why they work! Happy Science to you!