@ysmina-- That's a common problem. Before you start applying henna, take some sugar and lemon juice and mix it up in a small cup. Take a cotton ball and keep it ready. After applying the henna design, when you start seeing the henna drying, dip the cotton ball on the mixture and lightly apply it on the dry henna. Keep doing this to prevent the henna from drying.
Ideally, henna should be kept on skin overnight. Put a plastic bag over your hand and go to sleep that way to avoid a mess. In the morning, wash the henna off and apply an oil like almond oil on it. The henna design should be a dark red and should last for at least three weeks.