"Not new to" and "mastered" are two separate things - as I'm sure you know. It takes quite a bit of time to make technique your second nature. Until you are what I call "comfortably locked" into a technique (you're voice goes to it automatically, and doesn't fall out of proper technique) then you shouldn't attempt rasp. THAT being said, we all know you're going to anyway!

BUT I would wait until after you've done the seven week "boot camp" outlined in RYV (which will answer your question of "how often should I warm up"... which is EVERY DAY if possible, if you sing "regularly" it will become more natural and fluent.). Get your Vocal Stress Release done DAILY, OWN your Extreme Vocal Warm-Up exercises, MASTER your falsetto slides, Transcending Tones and Sirens. IDENTIFY AND SHIFT your core of resonance fluently. If you don't have these basic concepts under your belt then the "HOW" of rasp production is just going to completely confuse and escape you. I can tell you with great certainty that if you have ALL that stuff set and good, rasp will come very easily because you will have the knowledge and control you need to make it work - the RIGHT way. Think of your rasp like the roof of the house - if you don't build a solid foundation and sturdy walls, you're roof will crush and kill you. RYV and the 7 week boot camp are your foundation and sturdy walls.

BUT - like I said, You're going to try it anyway! It all begins on page 307 of Raise Your Voice 2nd Edition with Melissa Cross' explanation "Anatomy of a Scream". Since you've mastered the technique and build your strength and volume, smoothed out your vocal breaks, explored your resonance core, etc. It should all be pretty easy.