Look at your backyard and determine how much of it you are going to dedicate to a pond. Don't put it too close to the trees, or you will be skimming leaves out of it every day. It also should not be too close to drainage from the house, or it may get chemical runoff from the roof. Make sure the pond also gets several hours of direct sunlight each day.
Dig the pond using a shovel. Koi fish need a portion of the pond to be at least 5 feet deep. Goldfish and aquatic plants need at least 2 feet of depth; if you live in an area that freezes regularly during the winter, make it a little deeper to ensure the pond does not freeze over.
Install the necessary filters and pumps for your size pond. You will need a larger pump for a large pond than you will for a small pond. For example, if you have a 500-gallon pond, you should purchase a 500 GPH (gallon per hour) pump. This will circulate the water every hour. Follow the package instructions for installing filters and pumps.
Cover the bottom and sides of the pond with plastic lining. You do not want anything permanent for your lining, such as cement; if you decide you do not like the pond, you can simply take up the lining and refill the hole with soil.
Put any plants into the pond that you want. Koi do not need a lot of plants, but if you have goldfish, feel free to put more plants in the pond.
Fill the pond with water. You can use your garden hose to fill it. Add some detoxifying chemicals to neutralize the chemicals in the water. Follow the directions on the package; you will need a certain amount per gallon of water. Allow the treated water to sit for the recommended time.
Put your fish in the water. Do not overfill the pond with fish; they will die off quickly if crowded. Put in one fish per 100 gallons of water. If you put in a male and female, they will soon procreate and give you many more fish.