Crackle paint basing with PVA

Dumb question but i’ve yet to find a tutorial that actually explains this bit. So i’m messing about with martian ironearth. Got the base colours down and the varnish on now I want to try ‘thinned PVA’.

Do I paint a coat of this on first and let it dry? Do I use it as a medium? Do I put the martian iron earth on while its still wet. I want deep cracking but not flaking.

Thanks.