@feruze-- Don't put any plastic over the mehndi until it is completely dry. I recommend applying the mehndi at evening at the earliest so that it dries by the time you go to bed. And you can cover it with a plastic bag and sleep that way.
What I do after the mehndi dries is to apply a lemon sugar mixture on it. Just squeeze a little bit of lemon juice and add sugar in it. Apply this with a cotton ball after the mehndi has dried and repeat as it keeps drying. This makes the mehndi come out stronger and it won't dry and flake off your skin.
Put a plastic bag over each of your hands and sleep without touching anything as much as possible. In the morning, remove the bag and scrape off or wash off the mehndi. Follow with an application of oil on your hands- something like olive oil or almond oil. This will make the mehndi last even longer and the color will be a beautiful dark red.