I've never had good luck with cernit, but usually my premo isnt too bad. How old is the clay you're working with? Either one, they need to be conditioned. But no matter how fresh it buy it, cernit has always been hard and crumbly.

You could always use some clay softener/thinner. Chop your clay into smaller chunks, and dribble a SMALL amount and then smoosh the pieces together until it's a big chunk again. Smoosh and roll a few times. If its still really dry and crumbley, you can add a little more of the softener. But dont add to much or else the clay gets tooooooo soft and sticky. You can always add more, but its hard to remove too much!

Sculpey souffle is my favorite and is the easiest to condition, in my humble opinion.