do you want to attempt to raise many platy fry? if you do, you only need to separate the female from the rest of the tank. unfortunately this will stress her and depending on what the enclosure is, she may not want to give birth or not give birth correctly. If you try a breeder net, beware of when she starts to pace the sides. i have seen the female get a lip infection because she rubs her face on the net too much as she is frantically pacing the sides of the net/trap, trying to get into the main tank. If you can setup another tank, like a 10 or 5.5 gal ahead of time and cycle it, then she can be moved into that and things should go a lot smoother. just provide the fry with some floating plants to hide in from the parent.
If you decide not to move the mother at all, just put a whole mass of floating plants in the main tank. make them dense enough so that the other fish cannot go through, but fry can. when she gives birth, you will find some fry in the floating plants that you can net out and move to a rearing tank, or a breeders net (fry are not likely to get hurt in a breeders net/trap, but the adult female can like i mentioned earlier.) in any case always cover the breeding tank/net/trap so that fish cant jump out. some may try