The Guppy Breeding Tank
After about 20 days or so, the female(s) should be moved to a bare-bottomed breeder tank of 2.5 gallons to 5 gallons in size in order to receive the fry. While the gestation period is often about 28 days, it can range from 21 to roughly 40. This is dependent on a number of factors such as temperature and genetics
There are different ways to set this breeding tank up; however the most important aspect is that the water be at least 50% of her original tank water to avoid any physical stress. Usually, one makes sure there are many hiding places for the fry when they are born, using plants, etc. Another way of setting up the breeding area is to obtain some netting from a fabric store and drape it deeply into the tank, secured at the edges of the tank so that the female cannot make it's way outside of it. The holes in this netting should be large enough for the fry to swim through to the bottom of the tank. Yet another option is to assume the female is not cannibalistic and leave no space for the fry to hide. Thus, they do not scatter for safety, and the female does not prey on them. I do not recommend the latter for your first drop. Many females are cannibalistic.
A popular device called a plastic breeder box, sold at many pet stores, is about the right size for a bagel. A slotted bottom (try to choose the kind with a V-Shape for fry safety) provides an area for the fry to fall through. Breeder boxThe problems with this are: You do not know when to put the female into the box except while she is presently dropping; putting her in it too soon may cause premature birth/abortion; she will be unduly stressed; most healthy fry attempt to swim toward the surface of the water, and a trapped female will often grab the fry; fry may be hurt/hang in the slots. This item is best used for emergencies when a drop comes unexpectedly.
A female that is heavy with fry may become rather still, due to a cumbersome body, and more stationary yet, if she is moved to a strange tank. You may see her vent dilate with close observation just before the birth. You can usually tell when the female has finished birthing since she will stop dropping fry at the rate she has and will appear about as thin as you would want her to be.
She should be removed after birthing, and watch for any possible stress. Birthing is difficult for some females, and sometimes their bodies stop functioning normally. They may hover and become still and then lose the strength to swim. There is not much you can do in such a situation except do your best with the fry that were born.