We started training our girls when they would go into the play area outside the cage, we put a tray with the bedding and they would go to it as that is what was in the bottom of their cage. We then started taking all the bedding out of the cage bottom and using a tray with just the bedding, the rest was blankets, They took to it real easy and I hope this idea helps you as well.
Now ours live in a great CN cage and use the litter box all the time :0)