Circles? lol That's interesting.
You can sand forward and back if you want, but I'd work in an 'X' motion. It'll prevent any low/high spots from forming (unless they're already there on the body). Also, you really want to use a sanding block. Get a few different sizes to make it easier on yourself. You'll probably have to improvise to get all the curves of the car, as well. I used an old radiator hose and wrapped sandpaper around it to get in some of the grooves on the body and lines on the hood. You can use any spare hose or rag, though.
Tip: Mask off the edges around doors, windows, hood, hatch, etc. The sanding block will naturally want to stick on those areas more when you're sanding. That'll leave the bare metal exposed and vulnerable to flash rusting if you don't treat it.