I believe it depends on what kind of henna buy as to how you mix it.
I use rajasthani henna. I mix it with a hot black tea, to a thick pudding like consistency. I cover it, put it in a warm place for 6 hours. Then I may add pure clove essential oil, cajeput, eucalyptus and maybe orange for scent. I think the clove and cajeput helps to "terp" or stain better. The eucalypyus helps open the pours of the skin to stain better.
I always clean the area before I henna it. We are more dirty than what we think, even after a shower. I apply eucalyptus to my skin but some people who study aromatherpy say not to do this. I have no reaction.
That's my mix. I hope this helps.
I think some people may mix Jamilla henna with lemon juice and let sit overnight. Good luck.

Henna is great fun and really if you try a simple recipe it works well.
We use 1/4 cup of lemon juice mixed with 20 grams of henna , 1 1/2 tsp sugar and let it sit in a warm spot for 12 hours covered, and then and high quality essential oil. And more lemon juice if needed. Cover and let sit for 12 hours and then use.
This recipe works for us everyday of the week.
You need the acidity of the lemon juice, tea isn't acidic enough, but you might add regular coke and leave out the sugar .
Clove oil is not good to use, the terpene content is not very high and it is a skin irritant.
I am not aure what henna oil is, probably eucalyptus or something .
Here is more info for you, www.hennacaravan.com/how/how.html
good luck
Flavia

Henna oil is very dubious at best. Sometimes you can find out what's in it, but often you have no idea...so it can be good or not. I personally don't use henna oil because I want to know what I'm putting on my skin...that's just my "thing". I choose to use a combination of essentials oils (lavender, cajuput, tea tree oil, and/or geranium rose are the ones I most often use...the combination depends on what I have on hand) that I put directly into my henna mix and have great results. I personally don't use clove essential oil anymore because it is a known skin irritant.

***the essential oils, when you use ones that are high in terpenols (spelling?) react with the henna to give you an amazing, intense color. Are they necessary? Depends on what you're looking for. Good quality henna will stain when mixed with just hot water, but it probably won't be super dark and most likely won't last long. The different ingredients that you find in henna recipes are generally to help promote better color and/or longer lasting stains, including the essential oils.

I have used just black tea and gotten okay results, but lemon juice really makes a huge difference in color! I like to add sugar to get the paste stickier so it stays on the skin better.

I hope this helps, good luck on your henna adventure, it's a lot of fun!!!