Guppies are a member of a family of fish called livebearers. This family of fish is the easiest to breed. The females give live birth every 30 days or so. Many aquarists joke that the challenge with guppies isn't breeding them, but getting them to stop breeding. The only real effort in breeding guppies is protecting the young from their cannibalistic parents, or intensive selective breeding to produce and maintain various strains. Pond breeding is a good way to get many guppies quickly.
"Baby-proof" any filters. The best way of doing this is to put a sponge pre-filter over any inlet for filters and pumps to keep the baby guppies (called fry) safe.
Add fine-leafed water plants. This will give the fry a place to hide from their parents and improve their chances of survival. Anacharis, cambomba and hornwort are good, fast-growing choices. These plants can be tucked into the soil or left floating.
Keep the guppies well-fed with flake food and the occasional live or freeze-dried treat. Fish can eat an amount of food the size of their eye, once per day at least.
Monitor and maintain the pH of the water with a pH test kit. Specific test methods will vary depending on which brand/type of kit you use. Guppies breed best in neutral or slightly hard water, a pH of 7 to 7.5.
Remove adult guppies with a net regularly; guppies can breed so prolifically they can overrun a small pond within a year. You can add the adults to aquaria to sell or use as feeder fish.
Pond breeding can result in fish reverting to their natural colouration (drabber colours) since the breeding is not selective. Consider using aquaria for selective breeding smaller numbers of fish.
Do not release your guppies or plants into the wild, or any body of water connected to natural ponds, lakes or rivers. Guppies are highly invasive and will ruin any ecosystem they are introduced to.