Fill your Snow Blow Cannon bottle with 900mL of water and 100mL of Snow Job, attach it to the cannon and give it a good shake. Attach it to your pressure washer handle where the lance extension disconnects from the gun, turn the knob almost all the way to the "-" side, adjust the fan so you get a good arc, then move to the back of your vehicle. Starting at the bottom, begin foaming the car in a methodical overlapping motion, moving round the car till complete.
Allow the Snow Job to dwell for 3-5 minutes (less if you're in the sun) and then begin thoroughly pressure washing the foam away starting from the roof, and leaving the lower rear sections till last, giving the foam the maximum dwell time so that it can break down and remove as much grime as possible. Use overlapping sweeping motions, breaking the car down into sections (eg one door, one rear section).
More tips for using the Snow Blow Cannon can be found here.