Henna color and natural hair

[HeChangedMyName]

So, I have lots of gray strands and I wanted to use my henna that I have left from my relaxed days. I only want it for coloring purposes though. What is the best way to get the color to take? What should I add to it to make the color stick?

I have some red palm oil which is red and stains things, do you think it would be ok to add?

Also, if I remember, henna can leave the hair feeling really hard and dry initially. How can I combat this? I don't want my natural coils to be brittle and break.