I need your help!

I am attempting to make my Halloween costume this year. I usually do creepy make up and call it a day but this year I am trying to make the shrunken head from Beetlejuice.

I used a paper skull from the dollar store and some air clay to build around it. It keeps cracking! What am I doing wrong?

I didn’t use too much water, I didn’t spread it on too thick, do you have any tips?