Wearing safety glasses and a dust mask, mix mortar to the consistency of brownie batter. As with the concrete, mortar can be mixed by hand or with power tools. Use mortar to attach the stones at all points. Use a generous amount of mortar, especially at the center of the wall. Once a section is complete, use a wet sponge to clear away any excess mortar that has squeezed out between the stones. The mortar will be hidden on the completed wall, which will look as though it's held together by its own weight.