Remove the slipcover, and turn it inside out. Place the slipcover back over the chair. Gather the fabric along each seam where there appears to be excess fabric. Pin with straight pins. Start along the sides of the chair back. Continue around the seat. Often covers that are too large have seams below the actual seat corners. Pin so that the seams line up with the edges of the chair.
Remove the slipcover. If you discover that you can't get the slipcover over one area of the chair, adjust your pins to correct the problem. Sometimes this occurs when the top of a chair is wider than the lower part of the seat back. Your objective is to make the fit as close as possible while still allowing for the cover to slide on and off.
Remove the cover, and turn it right side out. If the manufacturer has attached tiebacks in the side seams of the chair back, make sure these tie backs are flat and pinned properly for your new seams. Areas of elastic may also be affected. Make any adjustments necessary so that your new seams will look crisp and even. Turn the cover inside out, and sew new seams along your pin lines.
Turn the cover right side out and slide it over the chair. The fit should be much better, even though you still have extra fabric bunching up the seams. If you are satisfied with the fit, turn the cover inside out, and trim the seams to a 1/2 inch. Iron the seams. Place the cover back on the chair, use the tiebacks and check the hem. Pin the hem evenly around the bottom of the chair leg. Remove, turn the cover inside out and hem.