If you want to use one of these breeder plastic thingies, just wait till the very last minute. Fish do go crazy in these things, but if you really would like to keep some of the babies, it will work.
By now, I usually know when my swordtails or platies are ready to give birth (the swordtails swim up and down in a corner, hover in the corner for a while and then go up and down again, the platies hide in the plants and move their fins really quickly). At that point I would stick them in the breeder (don't use a breeder anymore..). If you stick them in too early they not only go crazy, but most likely give birth way too early.
You need to sort of keep watch because even though the babies fall through the grates in the bottom, they can also swim back up and still get eaten by the mom! I would release the mom as soon as she gave birth to all the babies you want to keep (do you have a place for them though??).
Leave the babies in the breeder for about a week or so, then they will be big enough to swim in the bigger tank.
If you stick in a few plants and/or use one of the plastic floating plant-thingies, you will maybe also have some babies survive (this is actually currently my problem, too many babies survive in my community tanks, even though there are plenty fish/fry-eaters, so I am now "stuck" with two extra fry tanks...(anyone in need of a male swordtail, a bunch or lyretail gold mollies or some tri-color platies?)
L.