i do it either of these 2 ways:
firstly; get your tar a draw lines from the tyre across the side wall, do this with a 3-4 inch gap all around the rim, both sides. flip the bike upright cycle round with the brakes half on so the pads are rubbing, this wears the tar in.
secondly; flip the bike onto the bars, spin the cranks round so the wheel is spinning farely fast put the tar to the rim semi-gently. Do this both sides and then wear the tar in as above.
either of these methods should work.