How to Upholster Chair Corners ~~~ This tutorial details how to upholster a drop-in seat from scratch however if your seat base is still in good nic, you may simply need to recover one instead. If so, just follow my instructions beginning at step 7.
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!