First, let me thank you for your obvious will to take care of this old gal. There are so many stray cats in the US that it is very important for those of us so inclined to do as much as possible.
I'm assuming that you are already aclimating her to the indoors. Now, take it a step further and do something that may seem a bit mean, but really isn't.
Put her food, bedding, and a clean litter box in a room with a door that can be kept closed - a bathroom is best. Cats will not "evacuate" where they eat, so if you put her box as far away from her food as you can, she will more than likely begin using the box. Do this for about a week. If it works, you can let her roam the house. She will remember where her box is. After a month or so, if she is very good about using the box, you can move it to another location if you want, but be sure to show her where it is. And, get yourself a small wisk broom and mini-dustpan to keep the area neat. Clean the box of clumps daily.
If she has an accident on the carpet, clean it with plain water and use baking soda to get up the smell and the liquid. NEVER yell at her when she has accidents, but always praise and pet her when she does good.
Finally, be sure to get her fixed if she isn't already. A spayed/neutered animal is more likely to have good potty habits, even when teaching old animals new tricks.
Good luck to you! Contact me privately if you have more questions.