Soak your pruning blades in alcohol prior to making the first cut. This will kill any bacteria on the blades and prevent an infection from appearing after the cut.
Determine where you wish to cut the Massangeana's stalk. The plant is sturdy enough that you can cut it back to almost any height you wish, and it will continue growing.
Cut through the stalk with your pruning shears. The stalk will sprout a new crown of leaves from the point at which you made the cut, and will continue to grow from there. You can cut the plant just about any time of the year.