If the cat is in a relatively safe place, some people prefer to let them have the kittens and let her teach them the stuff they need to learn before trying to trap her and them.
The problem with this is that then it's not one cat you need to trap, it's lots. If you havea room in which you can foster her while she has the kittens, I'd try to trap her before she has them. It's not too harsh - she and her kittens will be safer for it.
I'd put out food for 2 - 3 days at the same time and the same place, and I'd only leave it out for 1/2 and hour. I would not wait too close so as to not scare her off from it. If you can find a large cardboard box, it'd be a good idea to cut the flaps off both ends of it and put the food in there. With both sides open, she should feel safe enough to go in - but it will also help her be comfortable with the idea that she's going "in" something to eat.
This should be enough time to get her coming. Then I'd put the trap out. I'd wash the trap and spray it with Feliway before I put the trap out. I'd cut open the cardboard box, and kind of prop it over the trap. I'd pour kitty litter or potting soil over the wires so she doesn't have to feel the wire on her feet. I'd put something really stinky in there, like tuna or herring (no sauce!). I'd have contacted a vet, so I could take her straight to the vet for a check-up, flea treatment (if necessary), &etc.
I'd put a big plastic tarp down over the seat of my car so if she's freaked (which most trapped cats are) and pees or poop, it doesn't ruin the seat of your car. I'd put a light blanket over the top of the trap (not the sides) during transporation.
I'd have gotten a room in my home ready for her - with hidey boxes to chose from, a litter box with clay litter in it, and food and water.
What a wonderful thing you want to do for this kitty!