I read tons and tons of recipes online before deciding to start with the simplest one. A little bit of lemon + as many cups of warm water as needed. Through my research at Mehandi and Henna Sooq I learned a lot of tips. The most interesting one to me was that you should not use boiling hot water under any circumstances. It can burn your henna and lead to an uneven, patchy color on your strands.
For my second batch I used the above recipe and I added some olive oil. This is probably the only time my hair can actually tolerate olive oil. I used it hoping that it would give me an easier rinse-out but I didn't really notice a difference between this and the first time. For once it the EVOO didn't make my hair a stringy tangled mess, which was a plus.
Sidenote, I seem to be a glutton for punishment when it comes to olive oil. My hair doesn't like like heavy oils. My hair doesn't like heavy oils. My hair doesn't like heavy oils. Maybe now it'll stick in my head.
My current batch is very simple. Henna, warm water and some silk amino acids. I love it!