Before applying henna on hands, it is better to do a patch test and check the possibility of allergic reaction, if any. You can use almost anything to apply the Mehndi, depending on the sophistication of your design, but the most commonly used are the toothpicks. You can also make a cone using a rigid plastic paper.
About 3 hours is a decent length of time for allowing the Mehndi paste to settle on your skin. You can also apply lemon juice with sugar to keep the paste on your skin for longer and not let it flake away.
Once dried simply flake off the dried Mehndi, do not wash the applied area for at least 12 hours. Eucalyptus oil, mustard oil can be applied after taking off henna for a darker shade.
So, all you girls out there who are waiting for their big day, enjoy the color Of henna and make your life as delightful and blissful as the henna adds to your henna adds to your wedding.