It is a GREAT thing your doing!!! Our neighborhood had a feral cat problem. We contacted a ferrel cat society locally. They have traps to set up. I agree with everything the other poster said about the feeding and where to set up.
It does take patience. We ended up capturing a majority of the kittens within a few days. However, the mama cat and 1 kitten refused to be captured. The mama cat got pregant in the process. That led to a few more kittens to capture eventually.
Please have the cat fixed ASAP! We were EXTREMELY lucky we found homes for ALL the kittens. Especially since I lived in a Homeowner's association!!!
Lock the kittens in a bathroom for a few days to accostom them to the inside world. They need exposure to humans as much as possible. Expect to be scratched! IT will take a few weeks but continous human contact will help.
Good luck! We were successful and so will you!!!!