Trimming your mulberry bush

You can cut down the branches as you go, as this forces new shoots from the base. It takes time, but they will develop shoots, much like a bamboo forest.  But if you shouldn't cut all the branches down until the end of the season if you want it to remain a bush. This forces the trunk to harden (you are artificially dwarfing it, like you do with a bonsai) and many new shoots will spring up next season. If you want it to be a tree, leave the central branch, and let it develop smaller branches off the knots.