I initially apply PVA glue and sand to the base so the crackle paint will have something to stick to, I found the crackle paint peels right off if applied directly to the plastic base.
I water down the paint and apply it heavily since it needs to seep into the deepest recesses, I actually use a mix of Kimera red oxide and black but any very dark brown should work
A heavy drybrush
Another heavy drybrush
A lighter drybrush of the white
Tint the colours to a yellow brown and give the earth a dirtier look
Final light drybrush of the bone white and even a few touches with a pure white