Since guppy fish are live bearers these are relatively easy to breed as compared to egg layers fishes.For breeding you will need a healthy pair of guppy fishes. If you are using one male per female then there may be chances that male will stress out females for breeding attempt.So in this case provide lots of hiding places for female in aquarium. Or you can put 3 females per male.Then in second case you may get 3 pregnant guppies.When males and females are present together then they will surely breed.Sometimes it is observed that females are already pregnant when you buy them from fish tank shop.
You can tell it by looking at belly if you find dark spot known as gravid spot then female guppy is already pregnant.
The fish tank should be 10 gallons or more.Set the temperature to 77 to 79 degrees Fahrenheit .Provide them protein rich food , live food will be the best choice but other will also do fairly well.Change the 25 % of water per week to encourage breeding.
The gestation period of female is about one month then she will release 10-40 babies in 1-2 hrs.The female are capable of giving birth for many months even in absence of males.When female is about to give birth then her belly will become thick and you can see gravid spot.Also you can notice some change in behaviour that is she may eat less food or move slowly.
If you are having only one main fish tank.
If you are having only one main fish tank .Then there may be high chances that the new born babies will get eaten by the mother or other fishes.However mother is not interested in eating till she give birth to all babies it may take about 1 hour.So you have further choices.
If you are having fairly large aquarium then you can provide lots of hiding places by adding fake or live plants.You can add java moss or java fern.Add floating type aquarium plants and remember the other fish should not be able to see or access the babies.
Remove the gravels before hand because fry may die if gravels fall on them.You can use spawning mop to provide hide out and caves made up of rock and marble chips.
If you want to go for another option there is 2 other equipment available in market known as breeding box and breeding net.Breeding box is plastic chamber in which you can put the pregnant female .While she give birth the babies enter to another chamber present at the bottom of it through the holes while mother guppy can not.Here you have to put the female while she is giving the birth or before hand.Here you need not to be present .
The other option is breeding net.It is box made up of net and it can be attached to the Wall of aquarium.You can put the females in it but you need to be present because here babies will be present with the mother.When she complete giving birth then move her to main tank.The breeding net will provide protection from adult fishes in same aquarium.The breeding net should be present where proper temperature and water circulation is maintained.
If you are having separate fish tank.
Then put the breeding pair in separate aquarium .When female give birth to babies after that remove back the female to main aquarium.The size of this separate aquarium must be 5 gallon or more.Remember not to add gravels and use fish tank sponge filter .If you are using any other powerful aquarium pump then they may get sucked up in fish tank pump or fish tank filter.So decrease their power or tight their ends.So they may not get sucked up.If you have moved the fry from main aquarium then add water of main aquarium and try to maintain same parameter as of main tank .Do water changes 50 per week is enough or 25% per 3-4 day.

Feeding and Care of Fry:
Feed the fry brine shrimps , newly hatched daphnia or you can give them hard boiled egg’s yolk the yellow portion of egg.Form a thick paste of egg yolk then add very tiny amount of it.Try not to add it in extra amount , if possible remove the remaining food from aquarium other wise it may spoil.But doing so don’t harm the babies.Or you cab crush the flake food then form a paste then give to guppy fry.Give them 4-5 times in a day but in very small amount.As i have told you earlier cover any suction device if present.Switch on the light for 14 to 16 hours only.Set the temperature to 80 degrees Fahrenheit.