I am a big chicken when it comes to things like this. I have developed problems with contact allergies in my "older" age and have been afraid to use hair coloring of any type. Your informative site answered all my questions and allayed all my fears.

This is after my 3rd henna treatment. These pics were taken the same day after I washed it all out. I used the following recipe:
100 grams of henna powder and a mildly acidic liquid mix.

Notes and References
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.
Mildly acidic liquid: 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
Amla: https://www.mehandi.com/Ancient-Sunrise-Amla-Powder-p/aml_01.htm
Henna time and temperature: https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix pp. 10-13
Henna color oxidizing ("rusting"): https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix/ pp.25-29
Chemical hair dye allergy: https://www.tapdancinglizard.com/henna-for-hair-2/ pp. 18-26

Video How-To
Going red with henna: https://vimeo.com/manage/videos/739464950
How to mix Ancient Sunrise Henna: https://vimeo.com/manage/videos/770522457a>
Henna Root Touch Up: https://vimeo.com/manage/videos/767016457