Put your female in a breeder box. I use one that's made by penn plax I believe. You attach an airline to it and the fry fall into the bottom, then are sucked into a seperate enclosure so the mother can't eat them. There are other types that have slits in the bottom that allow the fry to fall down through the slit and get away from the mother. Guppies will eat thier fry if given the chance sometimes, or not fed well.
Once your female has given birth put the fry in either a small tank with a filter (sponge filter will work) or put them in a net breeded that hangs on the tank. If you put them in a tank to themselves keep up with your water changes and make sure they get 8-12 hours a day of lighting. I don't know why but for me this seems to affect how well the fry grow and look as adults. Also keep up the water changes in your 15 to keep the fish healthy and happy. They can be fed crushed up flake food (which I feed my guppy fry) or you can buy livebearer fry food for them and they should be fed 4-8 times per day. Make sure they are in something that restricts the other tankmates from getting them. Net breeders are around 2 bucks and work well. When they are bigger than your biggest fishes mouth they can be taken from the net breeder and placed in the tank.
The female is also going to need a break after she has the fry. Keep her in the breeder after you remove the fry. She'll be weak and the male (if you have one) will harrass her. A couple of days secluded should be enough.
If you have a male and female guppy you can expect fry about every month. Guppies don't have a problem at all with producing fry.
If you're interested in breeding guppies make sure you breed the best looking and healthiest ones in your tank. This will assure that you don't end up with a bunch of weak and inferior guppies. Becareful though. A female can give birth a few times after she is seperated from the male. If you're looking for a certain color strain keep the females secluded from the males until you don't notice them getting bigger and taking on that pregnant look. Then introduce a female of your choice and male of you choice into a seperate tank and nature will take it course. Keep the fry together though and seperate them when you can distinquish which are males and females. This will help with inbreeding and allow you to pick and choose who gets to have fry and who doesn't.
Hope this helped.
