When I tried it with my boys they started just using the bottom floor (where their litter pan is) as a potty spot. I guessed that was better than nothing. But, then I moved their igloo to the bottom floor. And now they faithfully potty only in the box... Maybe I just got lucky with them. Or maybe it worked because there isn't a lot of floor space on the bottom floor hehe. But I think it really helps if you have a drastically different floor cover for the regular area to the litter pan. I use fleece cloth for the cage, and in the litter pan I use Critter Litter. It really helps with the smell too. I think the key is to designate areas for potty, sleeping, and eating. And just keep moving those poopoos to the litter pan and they will eventually get the picture. You just have to be consistent.
I picked the spot they went most (the bottom of their ramp) and gave them a big box full of their litter and switched to fabric on the rest of the cage. They adapted pretty quickly, just had to move the few mistakes we had into the box.
My other boys didn't adapt as fast, but they got it with some persistence.
I use Pets At Home safe bedding as the flooring for their cage and PAH wood litter pellets for their poopy tray. I think the main thing to really do is make sure you use a different type of bedding in the litter tray (As already mentioned).
Literally from the day that I had the girls, everytime they did a poop I'd pick it up and put it in their litter tray, along with any peed on bedding. They soon got the message and are well behaved girlies now