Add your entire desired dose to the pipe. Handling and messing with spice while under its effects is really challenging, and distracts you from your precious experience. It's difficult to take it all in one hit, so people take multiple -- 2, 3, 4 hits. Taking lower doses in one go is somewhat easier though.
Just add your desired dose to the pipe, heat it slowly and take as many hits as you want. Try to do it in less than 4 or 5 hits if possible, as I find that fewer stronger hits make the experience better and gets you farther. That's just me though.