Update: Below is the original chair cover instructable I did. I wanted to add a picture of the newest cover I made. It's a heavier deep red fabric and more attractive. It is sewn around the edges by machine very quickly and roughly. I inserted cheap and strong clothesline cording to cinch it and tie it to the chair pads. Very QUICK and easy!
I needed a quick cover for my expensive office chair. This took less than 45 minutes to make. You can use just about any kind of fabric. An old sheet would be great. I used a medium weight, Teflon coated, cotton upholstery fabric. I will just throw it out when it gets beat up or I get too sick of the pattern (sort of ugly). Perfection is not the aim here. I am going for function not fashion.
Step 1: Find fabric, clear 2" wide tape, cording and scissors.
Drape the fabric on the chair and estimate how much fabric you will need. Make sure you have fabric for a very wide hem to go underneath the seat and in the back of the chair. Your fabric should be approx 8 inches larger than the chair pad size.
Recklessly cut around the chair with the fabric draped on it. Take your fabric and place it on a flat surface, right side down.
Step 2: Grab your cord and make a casing
Lay the cording around the fabric shape a few inches in from the edge. Fold the fabric edge over the cording and secure it inside the fold with the clear tape. When you get to a curve just gather the folds naturally. Don't worry about how neat it looks and don't fuss with it. It won't show when its done.
Step 3: Cut the holes for the cord and finish taping it up.
Tape the fabric hem with the cording ( or old drawstring whatever works) laying inside it. When you get to the end decide what is the back or bottom of the chair cover, where you want to tie the drawstring. Cut two holes there about an inch or two apart, on the folded area of the fabric. Thread the cord ends through the holes and finish taping the edges.