It's an interesting experience, scanning your own hair...

Okay, here's how I got the color. I have naturally medium brown (wig shade 8, actually, don't ask why I know) that tints reddish in the sunlight. The colour here is from about seven months of hennaing my hair every month and a half or so. I use henna and a mildly acidic liquid.

It takes a lot of henna to do my hair (it's to my hips). I keep it all in a big, giant plastic freezer bag in the freezer, and scoop it out as needed. It take 4 heaping ice-cream scoops to get enough henna. I mix that with warm lemon juice (room temperature) and cover it with plastic wrap. Once there's dye release I slather it on, put a plastic disposable shower cap over that, and do homework or something.

Once it's been at least two hours and I'm starting to go crazy from having stuff on my head, I go and wash it out. I find that once you've gotten the largest bits out, putting conditioner of some sort helps.

Notes and references
Real People, Real Henna: General advice: https://www.mehandi.com/article-a/417.htm
Henna: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-s/121.htm
Don’t have fruit juice on hand? Keep some fruit acid powders in your cupboard:
https://www.mehandi.com/Fruit-Acid-for-henna-dye-release-s/299.htm
More info on mildly acidic mixes: https://www.tapdancinglizard.com/henna-and-acidic-mixes/
Ancient Sunrise henna for hair kits with everything included: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-Kits-s/124.htm

Video How-To
Going red with Ancient Sunrise henna: https://vimeo.com/manage/videos/739464950