Contact the local utility company and have them come out and determine whether or not there are underground power lines in the area where you wish to build the fieldstone patio. They will mark off any area where you're not allowed to build if this is the case. This step is crucial to maintaining the safety of you and those around you, so don't skip it.
Dig up the soil in your desired patio area, shoveling about six inches deep into the ground and clearing out any weeds. Apply some type of landscaping fabric on top of the soil to ensure weeds do not grow up through your fieldstones.
Spread a two-inch layer of gravel on top of the landscaping fabric. Use a two-by-four or other type of flat surface to smooth out the gravel until you have a nice even surface to work with. Use a tamper to pack down the gravel until it is firm. Add another inch or two of gravel and repeat the leveling and tamping process. Lay a flat board on top of the gravel and place a level on top. Your gravel should be leveling very slightly away from any building structures (like your home) to ensure proper drainage.
Lay down a two-inch layer of sand and smooth it out until it's even. Lay down your fieldstones and tap them with a rubber hammer until they are tightly secured and level. Allow a small amount of space between each of the fieldstones. Spread sand into the joints of each of the spaces and compact the sand tightly with a trowel. Allow the patio to set for several hours and add more sand as necessary.