I put them underneath, tack the crossmember plates and jack them into the bottom of the car.
If your doors are aligned well, open/close doors until you find the sweet spot. Weld them it in.
Do the other side.
It makes no real difference how you do it. Welded them in with the car upside down, sideways, on tires, on jackstands. Even took a laser pointer with a car on a rotisserie, right side up and upside down, nothing to even worry about either way. The tolerance they built these cars with are a looser than the town floozy...