Very easy and you can do it with the wheels and tires on.
The following will help you obtain an approximate ride height:
1 - With the car on level ground, measure your starting point from the ground to the bottom of each fender lip.
2 - Raise the car. It is easiest with all 4 wheels off of the ground.
3 - Mark the "rings" on the shocks with a sharpie and turn the rings down to take the car down the same number of turns all around, i.e. 3 full turns at each shock or 5 full turns at each shock etc. depending on how low you want the car. Don't forget to loosen the small set screw in the ring with the Allen wrench provided with the car.
4 - Lower the car to the ground.
5 - Drive it around the block to settle the suspension. You can roll it back and forth a bit but you'll get better results after a short ride.
6 - Check your new ride height at each fender to see how much you've lowered the car. When you get close to your goal, go to the frame per the ACR manual. This is a bit tougher to measure. I cut out some foam pieces to the correct ride heights for my car front and rear, smooth track.
7 - Once you get your ride height close, you can scale the car, make minor adjustments with the sway bars disconnected and check / adjust your alignment as necessary. Reconnect everything and get to the track!
The ACR manual will give you an idea of how much 1 full turn lowers the car front and rear.
Good luck,
Dan