You can easily rosin your erhu bow. To do this first take the cake of rosin and rub it back and forth on the one part of the bow until you get rosin powder on the hair of the bow. Now go the next part of the bow and do the same and keep doing it until you've done the outer part of the bow and then move on to inner bow. So hold its end with one hand and put it on your lap and rub the rosin on the bow hair with other hand. Stretch the bow by using your left hand when you are rosin the bow.