henna cracking?

That's normal, to keep henna paste moist so it doesn't crack and will keep staining skin for hours, you need to seal it or wrap it, or both. The easiest and often the best seal is just lemon juice and sugar, you basically make a thin syrup and dab it on with cotton balls or Q-tips. Spray hair gel works pretty well and is more convenient. There's other options, like white glue, latex, face masks, and other gluey things, but I'd stick with either lemon/sugar for the best stain or spray gel for ease. New Skin or other liquid bandage makes a henna seal so strong you can type with henna on your fingertips, but it's not great for sensitive skin (makes my skin red if I use a lot of it.) Seal henna when it stops looking shiny but before it cracks. For large designs that take a long time to finish, you may need to seal as you go. Seals not only keep moisture in but stick the henna in place so it won't get smeared or scuffed.

Wrapping henna is simpler, but not as effective without a seal. Paper tape is a good choice for sensitive skin and for areas like wrists and lower arms. Toilet paper and scotch tape or plastic wrap work just fine. Gently placing a loose sock or glove over the henna, or even a paper bag, can help keep in heat and some moisture. I personally don't like wraps without seals since it's so easy to mess up the design, but with a seal your henna turns out very dark when wrapped. Using cotton balls stuck onto a lemon/sugar seal will usually give great stains, but it's hard to remove so expect a sticky mess. The trick of wrapping is to keep heat in, but also to keep sweat minimal so your henna doesn't melt and run.