How To Litter Box Train A Stray Kitten?

I have a stray kitten that I adopted from the Humane Society. She was 12 weeks old and is now 16 weeks. She’s very smart and has a great personality, but she does not use her litter box at all.

My cat will let me know when she needs to go out by going into another room in my house, so it’s really not an issue for me because I can walk right in there with her…but my question is….how do you train a stray kitten to use their own litter box?