Measure your fireplace insert and construct a frame per the measurements. Be sure to check for level at each pass (Image 1). We used 2x4 lumber and OSB plywood to create our fireplace frame. After constructing a frame, attach any hoses or accessories to the fireplace (Image 2) and place it inside the new frame (Image 3). Drill screws into the frame to keep it in place.
Next, cover your frame with 30-lb. tar paper (Image 1). This will act as a moisture barrier for your fireplace. Attach the metal lath over the tar paper and onto the frame using a nail gun (Image 2). The metal lath will provide support for the stone that will be placed on top. Install the lath rough side up so that the mortar scratch-coat will adhere properly.
Mix your mortar according to the manufacturer's directions. Apply a generous amount of mortar to the metal lath using a trowel, pushing it upward onto the lath. Be sure to keep the top of the trowel angled, and the bottom flush with the surface until the lath has an even scratch coat. Let the mortar dry overnight.
Mix the mortar for your stones and spread a half an inch thick layer to the back of each stone. Place your stones beginning with corners (Image 1) and work inward. When placing the stones (Image 2), hold each firmly in place for 20 to 30 seconds before releasing. To prevent continuous joints and provide a more natural look, alternate placement of stones with different sizes and colors.