Me personally? I never use breeders for expectant mothers. I let them deliver in the tank, then if I want to save a few I scoop them up with a net. I then place them in a breeder or I put them in my quarantine tank (if unoccupied).
Most of the time, I just let nature take it's course and never save the fry. I provide plenty of plants, caves and hiding places for them. If they live, they live. If not, well then I gave them a fighting chance. I have 4 fry in a breeder right now. A Dalmatian Molly and 3 Mickey Mouse Platy fry. I saved them because I had room in one of my tanks for a few more fish.
Unless you want to become a breeder, then I suggest just letting the mother give birth in the tank and then save what you can find (they will be hidden in and/or under things).
As far as putting the mother in a trap/breeder, I'll leave that to the more experienced breeders here.
When my fish are ready to birth, the males will nip at her under-belly and swim below her in anticipation of a nice fry snack.