Place the powered henna into a ceramic, plastic, glass or stainless steel bowl.
Use warm liquid to mix the henna paste, either warm water or brewed tea.
Add a dash of lemon juice to oxidise the dye.
Blend well with a spoon until you have a thick paste the consistency of yoghurt.
This will be a messy mixture, and it will stain any surface it gets on. It may be a good idea to wear gloves, and you should immediately wipe the mixture off of anything you accidentally smear it on.
Cover the bowl with plastic wrap and leave in a warm place until the dye releases from the henna (at least 2 hours, or overnight for best results).