To get the best results from your henna there are a few guidelines.
1. Make sure that your skin is clean and free from any oils or lotions, moisturizers create a barrier and the henna can't stain as well through these barriers.
2. Don't smudge the design while its wet. Wherever henna touches the skin it stains, if its smeared, that's what the end result will look like too.
3. Leave the paste on as long as possible. At the very minimum the paste needs to be on for 4 hours, 6 hours is better, 8 hours is best, anything longer than that is unnecessary.
4. Keep warm. Henna loves moist heat! Drink tea, wear socks, keep your body warm while the paste is on and your henna will get nice and dark. Just beware of sweating, you don't want your henna paste to slide off!
5. Scrape, don't wash. When its time to take off the paste scrape it off with a spoon or a plastic card. If the paste really sticks on hard you can hold it over some steam to soften it up, or use some lemon juice or olive oil on a cotton ball to wipe away any stray bits. But whatever you do, DON'T use water.
6. Keep away from water for 24 hours. If you can keep your henna design out of water for the first day it will increase your chances of a nice dark stain.