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.