Answer:
Leave food out, he or she should come up and eat it, and after a couple of days of this you start sitting very far away, but so that the cat can see you, and sit there while he/she eats. After a couple of days of this, inch closer, and so on. You should eventually be close enough to stroke it. Extend your arm very very slowly and carefully. If the cat doesn't run, gently, slowly, carefully stroke the cat. After a couple days of this you should be able to pick it up, but don't go anywhere yet, just set it back down. Couple of days, then you can try and bring it home.
Or, if you don't want to do all that, you can put out a humane trap and put food and water in it. The cat should get in it, and then there you go. Good luck!