On the Billing page press Have a discount code?, enter the code and press Apply. The months to choose from then show the discounted total, for example "20% off 2 months", and the full price struck through. The discount applies to the next months you pay for, up to the number of months it gives; any further months are at the normal price.
The exact amount is shown before you press Pay. Only one discount applies at a time: a discount from the FleetKraft team replaces a code, and a code cannot be added while you have one. A discount is given once: months already paid with it are not given again, and the same code cannot be used twice.