Restrict the main stem to just one, removing all other vertical stems, and keep the base of the stem clear of growth as the plant gets taller. When the main stem gets to 10 feet, or whatever height you want it to be, cut out the top and just leave the laterals (side shoots) in place, removing the lower ones again if you really want it like a tree. Prune two thirds of the laterals back to half their length in August - do the same again in mid February. the plant will attempt to throw up more shoots from the base - remove these when you notice them.