Measure out the desired length and width of your flagstone patio. Write down the dimensions, and take the dimensions to a stone supply store. Ask someone to help you determine how much flagstone and other materials you will need based on the measurements you wrote down. In general, 1 ton of flagstone will be enough for around 100 square feet.
Mark off your building area by pounding a piece of scrap wood into the ground at each of your four corners. Tie a string along the outer border.
Dig out the entire area within the area you marked off with a shovel, around 7 inches deep. Save any patches of grass to use for patching areas of your lawn that may need it.
Install flexible edging around the inner border of your building area by lining the edging along the inner border and pounding metal stakes down into the ground through the holes in the edging.
Add a 3-inch layer of gravel throughout the entire bottom area.
Use a plate compactor or hand tamper, both of which can be rented from a home improvement store, to compact the gravel.
Pour a 3-inch layer of sand atop the gravel surface. Drag a piece of 2-by-4 inch wood across the surface of the sand to smooth it out.
Starting in a corner, set each flagstone down on top of the sand, one at a time. Wiggle the stones into place and leave approximately 1 inch of space between each flagstone. Use a rubber mallet, if necessary, to secure the stones.
Pour filler sand into the gaps between each of the flagstones. Once all cracks have been filled, spray a light coating of water onto the sand so it begins to harden. Once the sand dries, spray water onto it again and wait for it to dry once more, which will help set the sand.