Scrub the concrete with a hose and a wire scrub brush, or use a power washer if necessary. Apply concrete stain remover to remove any grease stains and rinse off the solution.
Patch any small pits and cracks with a putty knife and vinyl concrete patching compound.
Mix mortar from water and pre-made mortar mix. When ready, the mixture should be spreadable like brownie batter.
Add a ½-inch layer of mortar over the first section of concrete with a notched trowel, enough to set about four stones. Set the flagstones in the mortar, leaving as narrow a space as possible between the stones. Put a board across the first section and use equal force over the surface to embed them in the mortar.
Continue to set the flagstones in sections. Take off excess with the trowel, and wipe off drops with a moist cloth. Wait two days for the mortar to harden.
Sprinkle fine sand over the surface and press it between the spaces. Continue adding and pressing sand down until the sand joints are full at the top.