Guppies are extremely easy to breed. All you have to do is put male and a female together and they will breed. The female guppy will be pregnant for 20 to 40 days before giving birth to 20 to 30 fries. During gestation, the female's stomach will become bloated. A few days before giving birth, a normal female's gravid spot will turn black, this is because of the fries' eyes. The fries must be separated from the adults, as the adults will eat the fries. The fries will fully mature in 2 or more months, depending on the size of the fries' tank, the amount and quality of the food they were fed, and the temperature of the water. The females can breed again about 2 hours after giving birth. Female guppies can store the sperm of a male guppy, some of the sperm will be used to fertilize the female's after she has given birth.
So now you have a tank full of guppies, it's time to start selective breeding. Selective breeding is basically breeding the best of each sex and breeding them.
First, Select your favorite male and a similar female guppy, then breed them. Keep the fries separate from the other fries. After all of the fries grows up, choose the best male and breed it with it's mother. Keep choosing the best male in a batch of fries and breeding it with it's mother, grandmother, great grandmother, etc. If the female dies, choose a female from its daughters and continues breeding. Keep doing so until all the males from two consecutive batches look almost identical. What you have now is known as a pure strain.