Place the slipcover over the top of the chair and locate the point on the cover where the cover no longer fits. If it appears that the entire cover is too small, use a seam ripper to separate all of the seams. Open the hem if the cover is too short. Remove all the loose threads, then iron the pieces of the cover so that they are flat.
Drape the cover pieces on the chair facedown in the position where they should fit and pin the pieces together. If your cover has ample seam allowances you may be able to let the cover out through this process to a size that will fit. Allow for a 1/4-inch seam allowance on all seams.
Sew the cover back together along the pinned seams. Hem last after fitting the new cover over the chair and checking the hem height. You can also add material or replace material to allow for additional fabric portions. Remember to prewash any fabric you wish to use to prevent shrinkage in the future.