I use a mix of henna and a mildly acidic mix to make henna paste, and wait for dye release and mix in 20 to 50grams of indigo (depends on mood, i like those irregular blackish streaks).

I always start applying at the top, using a hairdresser's color brush to make sure I cover every streak, and then, once the top is covered, drop the brush and use my hands to smack it all over the rest, starting with a ponytail, and ending in some sort of muddy turban.

I cover it up with a plastic cap, then some foil to keep the heat, some old towel, and leave that on for 4-5 hours. The result is a very nice dark red on my naturally dark blonde hair, even my grays are covered nicely.

Notes and references
Real People, Real Henna: General advice: https://www.mehandi.com/article-a/417.htm
Henna indigo and cassia: https://www.mehandi.com/Henna-Indigo-Cassia-Powde...
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
Chemical hair dye allergy: https://www.tapdancinglizard.com/henna-for-hair-2...

Video How-Tos
Root touchup with henna: https://vimeo.com/manage/videos/255292964
How to mix Ancient Sunrise henna: https://vimeo.com/manage/videos/255292964