When using henna hair dying products try to keep the dye on your hair for as long as you can. The longer the dye is active on the hair, the better it will colour the hair. Your hair will also benefit more from the conditioning nature of the henna and will feel softer.
Apply the henna to the hair, wrap some clingfilm around your head to hold it in place and allow it to stay on your hair for between 1 and 2 hours. by wrapping clingfilm around your head you will be able to carry on with jobs etc without it getting in the way or making a mess.
Once you've had it on for as long as you can, simply rinse it out with water and a bit of shampoo. no need to condition.