You want the car on a level flat surface so that the holes in the ends of the sway bar will line up with the links. You can do this with both rear wheels on blocks or flat on the ground if you can reach the hardware, but the front to back direction does not need to be level.
Just make sure your car is not going to roll off or fall off whatever you have it lifted with while you are under it!
On the ends of your sway bar there will be a couple holes that the end links connect to. You use ONE of these holes depending on how stiff you want your rear suspension. The link points (holes) closest to the end of the bar give the least stiff control, while the hole closer to the sway bar pivot point give a stiffer ride. You can use one close and one end point for a "medium" setting as well. All these do is transfer wheel deflection from one side to the other of course, but the effect can be impressive. I suggest starting at the least stiff position, drive it this way for a week or two, then go to a stiffer setting and see if you like the difference. I'm running the fat BSH rear bar and prefer it on the soft setting.
