This is how I did the litter training with mine, it might be help for you:
Karl: I put a hay rack over his toilet, put newspapers on the floor of the cage and that was it. Every time he peed or pooped outside the litterbox, I would put it in the litterbox. On the 3rd day I thought he was never going to learn it, but on the 4th day he was perfectly litter trained and has been since. I should mention thought that he was caged during these days.
Molly: Molly was a hard nut to crack. I did the same thing (without the newspapers this time though) as with Karl: caged her, put poo’s and pee back into the litterbox. She seemed litter trained until I let her out the cage, then she forgot all about it. I had to bribe her to learn it. She was fed only in her litterbox, and I gave her a treat every time she was in the litterbox and both praise and treats when she in fact did her thing in the litterbox. It worked.
Good luck
I hope you can use some of this.