First, you should consider which color of henna you'll choose. Natural henna is VERY orange so I would really not recommend that on grey hair as it'll really look carrot. There are some darker blends for warm browns or even sort of a blue-darkish called 'indigo'. You can even mix different colored henna to make a special blend of your own. Here's a quick guide for the application: 1. Combine henna powder with warm water to a yogurt-like consistency. 2. Wash your hair with a clarifying shampoo to remove any oils or residue. Dry your hair slightly so it's damp, not wet. 3. Section your hair and apply the henna from roots to tips, ensuring every strand is covered. Use gloves to avoid staining your hands. 4. Cover your hair with plastic wrap or a shower cap to keep the henna moist. Leave it on for 2-4 hours (or overnight for richer color). 5. Rinse out the henna with water, then use a conditioner to remove any residual henna particles. The most important thing to keep in mind for a successful regular henna coloring is to avoid doing it too often. Henna buildup can be pretty nasty so you should be careful not to do more than 1 treatment a month or even every second month, depending if the color is still fine enough.