Now (springtime) is the time to begin that process if you have access to any willow. All you need is a branch or "whip" of willow. Cut the branch into segments 8-12" long, and each about a pencil's or finger's thickness. Stick the thicker end into the ground to about half its length, water and wait. The photographs show segments I planted last week. I hope to have a good willow patch by fall.
Each willow that grows will need to be trimmed back in the fall or spring. It will then grow back double or more.