Yup, what Don said!!!!! Get the car level side to side and front to rear. Left over bits of scrap, even paint sticks work good to level out the jack stands. Having everything nice and level will also make the install go better.

I usually also put some center punch marks in appropriate places inside the car for the cage to use as a reference when fitting the bars. The more secure and plumb you have the car, the better the install will go....