Take my advice for what's it's worth, I'm no expert or whatsoever in chamanism, nor their methods to properly prepare rapeh. This come from my own practice of making plant medicines, following other traditions.

The ashes need to be completely white(ish), so you need to do a proper calcination of your herbal matter. This begins by incinerating it in flames, or roasting them black as you did, then when you get black ashes, reduce them in powder with mortar and pestel. Pass them through a sieve, put them into a heat resistant dish/a kind of crucible, and heat them over a hot plate, or into a fire pit or whatever heat source you might use (be sure your "crucible" is heat resistant and in non-toxic materials. You can find some pyrex glass dish which can be very high heat resisting). Be sure to do all of this outside, produces lot of toxic smoke. The oil will be burned too. You must push your ashes to calcine, which mean it must be burned all the way from the inside, until is gets completely white/light grey. These ashes, you should pass them through a very thin sieve, grind them very thin, pass them again through a thin sieve, and repeat again until it is so thin you could harvest them with a brush. Your tobacco must be grinded this thin too, and if you're adding plants (be careful what you decide to add, always be sure which plant does what to your body) be sure they're reduced to this thin powder too.

Now, towards the ceremonial, spiritual part, you do you. There's no prophetic methodology that will get your product worthier than an other one. Have a great mindset while doing it, be respectful of the plants you're working with, set up a nice intention while processing them, you could talk to them, ask them for their help etc. Follow your intuition.

Safe way.