The first step in building a large pond is decided where you want to put it. Also, you’ll need a good inexpensive source of water. If you already have a well on your property, this will work well. After you dig out the area for the pond, you’ll need a way to seal it so the water doesn’t seep into the ground. For large ponds, many people use sodium bentonite to seal it. After that, decide what you want to stock your pond with. There are many choices, but do a little reading to find which kind of fish are compatible and which do best in pond conditions. Best of luck with your project!