litter-training a friendly feral

I don't know if this is the place for this question, but here goes.
We are currently caring for a cat that appeared on our doorstep. She's very affectionate and we were able to bring her in the house. She would spend the night and then out she'd go for the day, then come back after dark. Recently, she has been completely vetted (spayed/pregnant; negative tests; rabies and distemper shots). The problem is, she doesn't appear to be familiar with the litter box. I've been keeping her in, per post-op instruction. She recently relieved herself on the bed as if she had been holding it since she got home from the vet. We have three resident cats who have boxes in the basement, but we brought one up for her to use and it hasn't been touched. What to do?!

I had to litter train Nano so here is how you start...

Go to Walmart/Target/whatever and get a big bag of cheap topsoil/dirt to use instead of kitty litter. If you don't already have one, also get a low-lipped open box to serve as a makeshift litter box. Fill up the box with dirt and leave it in an area of your house where she seems comfortable spending time. Depending on her level of curiosity, she will either find this box on her own or you can pick her up and place her down in the dirt. If she has somewhat normal development, the urge to bury/hide her feces/urine will be a natural inclination and she will start using it at least part of the time. Then you slowly switch to the cheap clay litter...at some point you convert to a more traditional covered litter box...and finally you switch to your preferred brand of kitty litter.

Here were some things I came to realize when working with Nano that might or not be relevant to you:

Nano immediately understood the box could be used for poop but still used the bathtub to urinate. I finally got her to do both in a single box, but then she began escaping to go outside to defecate. After a lot of trial and error and suggestions, I came to realize Nano wanted two litterboxes -- one for each kind of business. For the last few months she has used the respective litter boxes with 100% reliability and no efforts to escape outside. I also found out that Nano expects a very deep layer of litter. I would have preferred to use the "crystal" stuff, but because she seems to really want about 30-40 lb of litter in her boxes, we only made it up to the clumping variety and that is how it will have to stay.

Funny story: The other day I went to the checkout lane with two boxes of 35 lb Arm & Hammer kitty litter. The cashier said "good lord, how many cats do you have?" and didn't believe me when I said "just one".

This is a trick that I shared repeatedly on this board for people who deal with strays and ferals, and it works. There is problem in getting cheap dirt or topsoil, and I would recommend that you not go that route, but instead get organic potting soil- because otherwise you will have to deal with bugs, specifically maggots and that is not very pleasant for you or the cat.

Also you want to be sure to keep these boxes- you should have two of them as clean as possible, for even organic material will attract bugs over time. But your cat needs two boxes not one, because if she were outside, she would never pee and poop in the same place, it isn't natural. Make sure the boxes are spaced far apart, not crammed next to each other

If she continues to pee out of the box, I would call your vet and take her in for a recheck (these are often free exams) It is quite possible she has stressed herself out and she is ill and if so this needs to be addressed-

Good luck!

Thanks for the suggestions. I will try the dirt-in-a-box (x2) and see how it goes. I'm concerned that the room she's most comfortable in is our bedroom and I'd rather put her in my office at the other end of the house. Should she be in there with the door closed? She likes to be close to us at night. Plus, we have 3 already established cats (one who really likes to dig).

If she has had so many life changes, I would let her stay in the bedroom for now. You can always slowly move the litterboxes after she gets more comfortable in using them, and living inside.

You are great to save this cat!