She sounds like she is feral, so you would need a trap first thing. You can usually get a humane cat trap from your local animal shelter, feed store or vet. Once you secure a trap, you can use the cheapest smelliest cat food you can find, or sardines work well. Dribble the juice all along the bottom of the cage and over the spring and put a shallow small bowl of food to the back of the cage, set it and wait out of sight.
Now you suddenly find yourself with this wild, spitting hissing feral. She is trapped in the trap and both of you are looking at each other from very different eyes. You are looking at her, thinking warm, fuzzy thoughts about lap naps, purring kitties, rubbing a soft tummy and she is looking at you with so much distrust in her eyes and thinking about dark corners, field mice, robbing garbage cans for food and she is wondering what the heck she did to deserve this sudden confinement?
So what do you do? First of all, you find her a spot in your home where she can stay safe. A quiet, dark, room away from any resident pets or small children. You want to be sure she has other spots in this room to go where she can feel safe if she feels threatened, and she will consider you a threat. After all, she has lived her entire life on her skill to survive, and one of those skills is how effectively she can hide. You will come to be surprised in the days ahead at how small this kitty can make herself and where she can hide from you in a room you swore you sealed up completely. So you provide her with a big cardboard box flipped over with holes cut in the sides big enough for her to pass through. You weight down the top of the box so she canâ€™t move it. You give her a nice big litterbox and mix just a little bit of outside dirt in with regular cat litter. Donâ€™t use clumping litter, not just yet. Use regular clay litter. Give her a big pan of water, a bowl for wet food and one for dry. Instead of using your regular lights in the room, use instead night-lites. Bring in a CD player and put on a classical piece really low on endless play. Fetch up some old rags, or a blanket, something soft that as soon as she figures out the hard ground isnâ€™t that comfortable she will seek out and use.
Now step back and survey the room. The first thing she is going to do when you let her out is run. So check shelves and counters if there are any and make sure she has a clear landing pad should she decide to go ballistic and run amok. If you want to give her air and open a window, you can go to any hardware store and buy a piece of decorative trellis and cut it down to fit the screen opening. Through the slits in this decorative piece the air will come through, but she will be unable to escape if she decides to press against the window and push the screen out. You can also used chicken wire, but the trellis looks better.
Okay, now you are ready for her arrival, so go and fetch her, and take her into the room, shut the door and turn her loose. Donâ€™t go after her, watch where she goes though, then step out of the room and LEAVE HER BE.
Just let her get used to the room, donâ€™t be going in to peek in on her for at least a day. She has food, water, a safe place to stay and so leave her alone for 24 hrs at least. Once you have done this begin to visit with her at scheduled times during the day. Sit on the floor and read softly to her, provide her with a big box that is high enough for her to get into but also high enough for the kitties to NOT fall out of. Spread kitty treats around you as you sit on the floor, bring in a peacock feather or something like that to play with her if she will let you. Chances are she will hide from you so you just want to get her used to your voice. Also take an old shirt of yours that you don't care for and do a work-out in it that would make Richard Simmons envious, get this t-shirt soaked with your sweat, then peel it off and leave it in the area that she is sleeping in.
If she does venture out, just don't stare her in the face, let her make the moves on you and gain her trust. It doesn't happen overnight, but it does happen. The main thing is to get her trapped before the kitties arrive and keep her away from any Toms that will want to hurt her kittens. Good luck!