stand it in water, they root really easily. When you see strong roots at a decent length pot into sturdy pots with a stick to tie the stem to as it grows. It's best to do this in the early summer but I've just potted 2 stems which rooted in water. They got broken off in strong winds a few weeks ago and were about 18" long.
Some say you get stronger roots by putting cuttings in compost but I like to see the roots grow and have never had a problem.