I got a rectangle table cloth. I didn't measure anything before hand so I just got one, hoping it would be big enough and it was plenty large. I have extra in case I'll ever have to redo them.
The first step is to remove the chair seats. This normally involves only a few screws on the bottom. Some of mine were a little tough to get out but as log as you get pretty much all but one it should come off.
Then I laid it on the back of the table cloth and drew where I needed to cut to make sure there was enough overhang that I could fold over and staple. I folded the sides with the screw holes first, other wise there was too much material to go through. I poked the holes got some of the extra fabric out of the way so it would be easier to screw back on. Then I folded it up sort of like you would a present and staple away using my husbands stapler. Then screwed it back on.
So much nicer. And they are pretty well stain resistant because you can just wipe them clean when something spills and you don't have to worry about it soaking in.
The down side is that my cat likes to climb on them and we haven't gotten her declawed yet so one of them has a few little holes but you can't see them. Plus, if they do need to be replaced again, I have plenty of left over table cloth. and we can change them with another inexpensive one if we decided to. So much better than buying new chairs!