Ok what you do is to lay the fabric wrong sides up onto the chairs. You pin the "seams" Once you have the parts pinned and all on you can mark the seam that will be open and I usually mark all of them. then you unpin the opening seam and take the thing to the machine and sew your seams. once you have your seams sewn you can turn it inside out and you have perfectly fitting slip covers. Watch this video. It will explain it better then I can.
http://www.youtube.com/watch?v=bSG0bEnMgrY