In this sub-phase, you'll choose one of your units and declare the charge you want it to make. Your opponent will then have the chance to have their unit hold its ground or react to the charge, either by holding, standing and shooting, or fleeing.

Once the charge reaction has been resolved, you can nominate another of your units to declare a charge, and so on, until all of your charges have been declared and reacted to – then you get to make your charge rolls and resolve the charges.