Here is a great article with some tips that might help
Also a 3 part video on working with feral kittens
And finally some general tips and techniques to help:
1. The first tip is to have a room that is dedicated to the cat. If possible this should be a room where the cat will be confined for a period of time. You can use a spare bedroom or even a small bathroom. If using a bedroom or other room of your home, be sure that the room is cat proofed. Remove mattress and box springs or place them directly on the floor. Hiding under a bed is the first place the cat will run and it is almost impossible to get them out. Also block behind dressers or book cases. Cats can fit into very small spaces. You do want to have an appropriate hiding spot for the cat. This can be an old box turned on its side or a hiding box found on a cat tree. Also make sure all windows are tightly closed and blinds are up with the cords hidden. Do not be surprised if the cat throws himself against the window in an attempt to escape.
2. If you do not have an empty room available or the cat is very wild, you can use a large dog crate or fasten two smaller crates together. Just be sure there is enough room for a small litter pan and food. If you need to use a crate, I would suggest keeping the crate covered with a sheet or towel when the cat is alone. This will help to calm the cat. Eventually you will need a cat proofed space where you can move the cat.
3. You will want to start with 2 litter boxes. Many feral cats are not used to urinating and defecating in the same location. When choosing a litter, you can use fresh clean potting soil mixed with non clumping litter or you can try Dr. Elsey’s Litter Attract. I have found the Litter Attract to work very well. If the cat has an accident on the floor, sop up the urine with a paper towel and bury it into the litter box. Do the same with any stool. Make sure you clean the area very well with a good enzymatic cleaner to remove all traces of odor. Place the litter boxes away from the food and water.
4. Feliway plugins are a great way to help a nervous cat adjust. Most cats find the product soothing. You can find Feliway at most pet stores as well as on Amazon.
5. Another product I found helpful was Composure treats or Composure Liquid Max. The first few nights inside are very stressful. The cat will often yowl and cry all night. The Composure is very useful in calming the cat at night. I first tried the treats, but after a week or so, the cat no longer liked the treat. I then switched to the liquid as it was very easy to mix into wet food. Composure is available at the best price on Amazon.
6. No direct eye contact. Feral cats find this very threatening. Keep your gaze over the top of their head or look down.
7. Talk softly. You want them to get used to your voice. Carry on a conversation or even read aloud from a book or magazine.
8. Sit on the ground so you are not looming over the cat. You also want to keep your movements slow.
9. Food rewards are the way to a ferals heart. Find a special yummy treat such as plain cooked chicken or turkey, salmon, tuna or sardines. I would start by giving them a very small plate with some of the special treat. I would then have small pieces beside me and begin to toss the treat towards the cat. Each toss I would get the treat a little closer to me. I would do this activity each day. After a few weeks, the cats are often getting quite close as long as you keep your movements slow and gaze down.
A very special food reward I stumbled across is Gerber Stage 2 baby food. Cats love the chicken or turkey. The jar is very small with a blue label. The ingredients are chicken or turkey and water, no added spices. I started with a bit on a plate and as time went by, I offered it on a spoon. When working with young ferals or kittens, they can often be coaxed to lick this off your fingers. Just remember this is a treat reward and not a meal replacement.
10. Remember that when working with ferals it is often 1 step forward and 2 steps back. Just take it slow and steady. Also celebrate the small advancements.
11. Jackson Galaxy of Animal Planets, “My Cat From Hell” series is the cat guru. He has a line of flower essences that are specially designed for cat behaviors. He even has one for feral cats. I have used this essence with great success on all of my feral cats.