Take the car somewhere flat.
I just do it by eye. Wind it in/out a bit until the wheels looks straight ahead or have a bit of toe depending on what I want, when you lower it down to the ground. Small adjustments. Repeat until happy. Get someone in the car to shake the steering wheel until the wheels are straight ahead rather than you sitting in and making the steering wheel level, incase that's out.
Good opportunity to centre the rack whilst you are there also.