The ground area will need to staked off and leveled with a shovel. Cut posts and set them into the ground at least a foot deep. In cold climates, dig the postholes three feet down to get below the frost line. This will prevent movement during freeze-and-thaw cycles.
Once the posts are plumb, lumber and clamps can hold posts in place while quick-setting concrete hardens. When using quick-setting concrete, pour dry mortar into the hole, then fill it with water from a garden hose and let it cure overnight. Each 3-foot hole requires five bags of concrete.