Sand the banister lightly, to roughen the surface so it will accept the paint easier.
Paint the banister with the light brown paint. Let it dry at least 24 hours. Paint a second coat of light brown paint, and let it dry at least 24 hours.
Skim a brush with the dark brown paint over the banister, so that the light brown shows underneath it. Pull the rocker through this coat while the paint is still wet, moving your wrist back and forth so that the design created mimics wood grain. Use paper towels to clean the rocker after each stroke.
Dip a brush into the dark brown paint, then wipe off most of the paint with a paper towel. Lightly brush the banister, taking care each stroke moves in the same direction to further mimic wood grain. Let dry for at least 24 hours.
Apply a coat of varnish to the banister, and let it dry completely.