The rottenstone (dust) should come off of all but the recessive areas. RS is applied after the tone, wash, has been applied, wiped, or other wise worked and then dry. Dust the Rs on and rub off of the high spots, leaving the RS in the recessive areas. It should have enough adhesion to withstand light brushing. Depending on the look I'm trying to achieve, I may use compressed air to remove more of it. On an older frame, you will need a very light tone, mineral spirits and a touch of oil or oil color, washed on, wiped, dry, then the RS, buffed off. I'm hesitant to use pumice as it's a coarser abrasive than RS.
That's the quick, 5 minute course, in something I've been "playing" with since I was in art school, a while back, and geared to what materials it sounds as if you have. There are multiple variations of any and all aspects of the above.