Choose a waterproof, lightweight, sturdy string for your prism. Fishing line is a good material, as it repels water and allows your prism to spin freely.
Cut at least a foot of string and fold it in half. Thread the folded end into your prism, and pull the other end of the string through the folded "loop" to create a sturdy knot.
Find a sunny spot to hang your prism. Sunlight must flow into your prism to create the desired rainbow patterns. If you hang your prism in a shady spot, it may never catch the sun.
Select a structure to hang your prism on so that it doesn't blow around, bang into things and break. This could be a trellis or a sturdy branch.
Securely tie the string to the selected structure.