how to apply indigo powder after henna - #25455

I am trying to figure out how to apply indigo powder after henna and feeling a bit confused about the whole process. Last week, I decided to do a henna treatment because my hair was looking dull and lifeless, and honestly, I’ve heard so much positive stuff about henna! The color came out beautiful, but now I really want that deep, darker shade that indigo offers. I’ve read that you need to let the henna sit for a while before mixing in the indigo, but like, how exactly do I do that? Do I just mix the indigo with water, or should I add something else? And for how long do I keep the indigo in? I really don’t want to mess this up, but info on how to apply indigo powder after henna seems kinda scattered everywhere. Oh! And did I mention my hair is super, super thick? That makes it even more tricky! I’m also worried about how to maintain color afterwards... any tips on that? Thanks for helping me out!