Upholstering Instructions at Better Homes and Gardens. In a nutshell: Strip the chair of its original fabric and use the pieces as pattern. Then pin the pieces wrong sides out onto the chair. Remove pinned cover and sew it together, with welting reinforcing the seams. Put sewn cover back on the chair. It should fit like a glove!
OMG YOU HAVE to CLICK to the original Link on this one-- this was a HORRENDOUS old office chair (I would have totally ditched) but WOW did Kendra at My Insanity have better ideas for it! Kuddo's Kendra colorful-office-chair-4