Harvest hair from your hairbrush!  You won't know what henna is going to do to your hair until you put henna on your hair.  Strand tests don't do the job.  Collect hair from your brush, or from your shower drain.   Everybody has slightly different hair!  No two people are going to get the same results.  
Mix up a henna paste that you think might work.  Look at what other people have done with their henna and hair at MixesOrder some samples and try different mixes. 
When you've got your henna mixed up, smoosh your harvested hair into the henna mix.  Stir it around until there's henna all over the hair.  Save some extra hair to compare with the hennaed hair.
Leave the hair in the mix a few hours.  Rinse out the lump of hair.  The color after rinsing is NOT the final color!  It takes both indigo and henna 2 or 3 days to oxidize and settle down to the final color.  

3 days after rinsing the paste out of your hair, see if you like the results. Compare it to the hair without henna.