Wash the wood with warm water and a few drops of dish soap to remove any dirt or debris that interferes with the darkening process. Use minimal water to avoid damaging the wood's surface. Wipe away any dirt with a damp sponge and then dry the piece with a towel.
Sand the wood with 220-grit sandpaper. This helps the new finish bond with the old one. Wipe away any loose sand with a towel once you're finished.
Apply a tinted varnish to the wood with a paintbrush to darken it. Apply additional coats once the first coat dries as needed since each application darkens the wood a little more.