Put the slipcover on top of your chair or sofa. Make sure it is centered with an equal amount of fabric on each side. Make sure the front bottom edge is straight. If anything looks uneven, adjust it by gently pulling the fabric before continuing.
Tuck the slipcover's corners in around the cushions, and smooth out wrinkles on the arms and covers.
Pin the slipcover's fabric folds down, using quilter's pins, anywhere the fabric overlaps or drapes. Place the pins about 3 inches apart so you can see how the cover will look when you are done.
Thread the hand-sewing needle and knot the heavy-gauge thread ends. Slide the curved needle's tip under the edge of the overlapped slipcover fabric. Stitch the folded edge of the overlap, using a 1/8-inch long slip stitch or blind stitch.
Sew all the remaining pinned areas. Remove the pins. Smooth the cover, using your hands, to remove any remaining wrinkles. If you notice any new overlaps, pin and sew them following the same sewing steps you used for the other overlaps.