Before starting this project, you will need to felt an old 100 percent wool sweater. Simply wash the sweater in a washing machine with plenty of detergent and hot water, and dry in a clothes dryer on high heat.
The soap, agitation, and heat will cause the scales of the wool fibers to open up, lock together, and condense. Washing the sweater with something made of heavy cloth, like a pair of jeans, will speed the process.
You may need to wash and dry the sweater a few times -- every sweater's different. Test your sweater for felting by snipping the edge -- it should not fray. If it does, wash and dry again.