Sometimes catnip will make an angry kitty even angrier, so be mindful of that.
Try to borrow a hav-a-heart trap from your local humane society. That is a trap in which the door closes automatically. Much better and safer both for you and the kitty.
Best thing is to let them come to you on their own, but if you use a trap, get a hav-a-heart.
The kitten you're trying to trap sees you as a big, mean predator who ate his/her brother/sister. This kitten will be very
wary, and won't want to come near you.
After s/he is inside your home, provide food, water, and a litterbox for the little ones.
Let them get accustomed to you in their own time. Slow, soft movements are the rule around these babies. Don't make sudden predator-like moves.
And complete quiet, or very soft music. Remember, their ears are much more sensitive than ours.
It takes some work and patience, but ferals make the best, smartest, and most loyal companions.
Take it from folks who know.