By training cannonneers first or hire them from towns. For example, Spain will be able to use more stronger cannons than the Flemish, but they have more accurate etc. So it also depends on what kind of cannoneers you have because they can only operate with a certain kind of artillery.
You can build cannons from the camp menu. I am planning to make it quite expensive so you cannot spam directly around with artillery. Maybe I will try to get cannons on another way by for example completing a quest for a Mayor of a town who grants you the privellege to buy cannons from that town or something.
Because building your own cannon's from scratch wouldn't be really realistic.