To rosin your bow, you simply rub the rosin along the full length. Ten or twenty times should be enough. Horsehair has tiny fibres and the rosin helps these fibres stand out so they grip onto the strings.
Ironically the rosin is not good for the fiddle. After a while too much rosin will be left on the string and the fiddle may start to squeek. The string have to be cleaned with a dry cotton rag. Make sure your dog or cat is nearby, as they love the squeeky sound of cleaning excess rosin from your strings.
You should also clean the rosin from the fiddle itself and the rosin can harm the finish.