I remove yellowing on fabric washing it with oxiclean powder even if the fabric is stitched it doesn't discolor either the fabric or the stitching. You may have to wash it twice.
dye the fabric if you haven't yet stitched on it, if stitched and the stain is just on the fabric use a stain remover, if it's been stitched and you know the threads are colour fast put it in the washisng machine, and wash at 30 degrees. If putting in machine scares the living daylights out of you, put it into an old clean pillowcase first, wash on 30 degrees :O)
I soak in a bucket of water with 1 tbs ammonia for about 20 min. Wring out and rinse well and check it. For stains on finished works rubbing alcohol on the stain itself with a q-tip, sometimes works too.