Determine a suitable location for your farm pond. Fortunately, good earth for ponds is bad earth for agriculture: You'll be looking for soil that has heavy clay content and poor drainage qualities. Ideally, you want your pond to be located in a very low point on your property so that it collects runoff from rains. In this way, your pond may help solve certain water drainage problems on your land.
Consider the uses you'll have for your pond before beginning, as you'll want to dig the pond to a suitable depth for your purposes. If you want to stock the pond with fish, you'll need to dig it to a depth of 12 to 15 feet. If you just want a shallow water reservoir for cattle hydration and water collection, 3 to 5 feet should do.
Use the bulldozer to push the earth from the highest point of the pond down to the lowest point. By damming your pond this way, you'll double your efforts by creating a natural bank on the high side when you remove the earth and building up the low side with it. Removing 6 feet from the high point and building up the low point can thus create a pond with a 12-foot depth. Otherwise, you'd have to dig straight down 12 feet to get similar results.
Dig the pond a foot or so deeper than you ultimately desire, and then evenly disperse several truckloads of clay soil throughout the pond bed. You can skip this step if you have particularly high amounts of clay naturally present in your soil. If you do not, however, this will significantly aid water retention over the long term. Make sure to compact the clay thoroughly after addition so as to compact the surface.
Plant deep-rooting grasses and trees on the backside of the dam to prevent erosion. Depending on the depth and size of the dam, you may also want to reinforce the base on the backside with heavy boulders delivered from a local quarry or building development.