With your ribs trimmed and your smoker hot, its time to put on the rub. If you apply the rub too early your ribs will get a "hammy" flavor. By applying the rub closer to the cooking time you will get all the flavor without the texture of the meat being changed by the salt and spices you find in rubs. Of course, if you prefer you can put the rub on earlier, but I suggest you experiment with it first.
The rub should be applied to the whole surface of the rack of ribs, thick enough to heavily coat the meat. Only so much spice will stick to the ribs and that's how much you want, as much as will adhere to the meat.
From this point forward you want to be careful how you handle the rack of ribs. The more you handle the ribs the more rub will fall off.
I have many great Rub Recipes that you can look through to find the one you like best.