Look for a zipper in the sagging cushion. If there is no zipper, carefully open a seam on the back of the cushion using a seam ripper.
Remove the old cushion form from inside the leather. Measure the height, length and width.
Purchase a new pillow form, or foam padding, in the same size as the one you are replacing. Find pillow forms or padding at craft stores or sewing supply shops.
Insert the new foam pad into the leather cushion, carefully pushing and adjusting until the pad is positioned correctly.
Zip the cushion shut. If there is no zipper, sew it closed. Tuck the seam edges inside and hold the opening shut with clamps used for holding large stacks of paper. Use a curved leather needle and heavy thread to stitch the opening closed. Make small, neat stitches and sew over the stitches for extra strength.