Keep in mind that mature guppy can also feast on the small babies.
If you are really set on breeding guppy you need a second tank and a ... what is it called in english... breeding trap?... Basicly you place the female in the trap(it's like a small plastic thing that goes into the tank...) and you let her do her thing, thing is done and you remove the female and just take care of the small babies till they grow and there you have it.
You can feed the babies boiled eggs(you need to make a ... paste out of them and just "wash" it in the water but not to much or the water will go bad!) and after they grow a bit you can use hikari tropical micro pellets.
Here is a perfect breeding trap:
Water should be at 76-82F, gestation period is about 30 days(more or less...).
Keep in mind that 1 female can give birth to up to 200 fry! Make sure you have something to do with them before breeding.