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The shims will help hold the panel in place, and they will ensure an even placement of the panel in the space. The shims should be about 1/2" (1.27cm) wide and long enough to easily remove once the first round of mortar sets. Install the shims at 3" (7.62cm) intervals, moving away from the corners. You can mix batches about five or six trowel amounts at a time. It should ideally have a consistency close to bread dough, which will stop the panel from floating in the space as its sets. This should just be a thin layer between the shims to help form a base before placing the window. The panel will be heavy, so have assistance on hand to place the panel. Reset any shims that move while setting the panel. Assure the window is plumb and square before continuing. Use a trowel and pack the gaps around the shims at the bottom of the panel. Allow it to set, hard to the touch, before mortaring the sides. From inside, scrape away any clumps of mortar with your trowel. Assure that the mortar is firmly set before applying pressure to the window. Slide all of your shims out and finish mortaring the gaps once the mortar has set. Allow the mortar to set for two hours and then smooth it out with a striking tool. Clean any excess mortar off the panel with a sponge while it is still wet. Applying mortar at the top gap of the window can produce downward pressure as it sets that can compress and crack the glass. Let the mortar set for a full twenty-hour hours, then fill the entire gap with 100% silicone caulking.
Install cedar shims at the bottom of the frame. Mix small batches of mortar. Apply a small amount of mortar to the bottom of the frame. Tilt or slide the panel into position over the shims. Slide shims into the top of the window until it is snug. Pack the gaps at the bottom with mortar. Mortar the sides of the panel. Apply caulk to seal the top of the window.