After applying henna, try to avoid using or washing hands in water for at least 6-7 hours as it will make the mehendi lighter.
Remove your mehendi with mustard oil instead of water to get the darkest mehendi on your hand.
You can also use cloves to darken the mehendi's stain, bake 4-5 cloves, and crush them to mix it into your mehendi paste.
Mix lemon juice and sugar and apply it on the henna with cotton.