Apply moisture barrier such as house wrap to the exterior wall. Doing this will help protect the frame from water and keep the back of the stone veneer from being damaged.
Apply a metal lath with galvanized nails or staples according to your local building code. This supports the mortar and provides a firm surface for the stones to attach to.
Apply scratch coat, which is a sand and cement mortar mix, over the metal lath. Make sure the lath is completely covered with a thin level layer. Using a mason’s scratcher, scratch up the smooth. Most codes require the coat to dry 48 hours before applying the stone.
Apply roughly 1/2" to 3/4” of mortar to the back of each stone piece, using a trowel, covering the entire back. Butter one piece at a time, just prior to applying the stone piece. Starting at the top of the wall push the stone firmly into the wall. Push firmly enough so that mortar is squeezed out around the sides of the stone.
If you plan to grout around the stone, us a grout bag. Much like icing a cake, fill the bag half way with mortar and on the corner of the bag cut a hole. Roll the top part of the bag down forcing the mortar out of the bag and into the joints. The hole in the bag should be approximately 5/8” wide. Make sure all joints and stone surfaces are left clean of excess mortar.
Note: If sliding or slipping occurs, the mortar may be too thin, or perhaps you used too little or too much