Mission's website, under "support" "FAQ's" recommends applying the rail lube every 15 to 20 shots. The lube is meant to protect the string, so you need to apply the lube to the flat tops of the rail over which the string rides during cocking and shooting. I was also told to apply the lube to the outside vertical faces of the rail to reduce the noise created by my cocking rope hooks rubbing on the rail as I cock it. As for how much to use, use just enough to coat the surface, smooth it out with your fingertip if you need to. Any excess will only run off, so no point in using too much. You'll figure it out by trial and error.