Removing the window casing
The window itself should be left in place. The inner casings need to be padded out, leveled and set flush so that once your initial “box” shelf is inserted into the hole; it fits snugly and securely without any wobble that can make any table decor fall to its doom. Use a level to find the exact measurement of the pad wood you will nail around the inner edges of the windows. Some casing may need to be removed, in other cases; wood must be added to make the inner casing of the window flush and measurements consistent. Remove any casing around the wall of the window as well.
Insulating the window and around the shelf box is crucial to ensuring your house stays snug and warm or cool and dry during the extreme weather months of the year. Fiberglass insulation works best, but ridged foam panels work just as well, so long as they fit. Never use expanding spray foam as it can easily expand and break the glass or push your shelf system out of level, making table decorations and other décor slide to one side of your shelf.
Building the box
Now that your window is insulated and padded so that an even measurement can be taken both horizontally and vertically, you can cut the inside edges and back panel of the shelving box. Cut two boards the same length for the horizontal members. Now cut the two vertical members three inches short so they all fit together with the vertical member sandwiched in the middle of the horizontal members. Attach them all together. Cut a piece of 3/8 inch veneer to fit the back of the shelf. Use the factory edges of the veneer to ensure you get a square cut. Now attach the shelf box to the veneer to create a perfectly square shelving box, almost ready for your wall art and décor.
Fitting and Shelving
Fit the box into the hole and assess that it fits snugly, is level and flush with the exterior of the walls. Shim as needed. Once you're sure of a good fit, you can easily pull the shelf back out and add a shelving system as needed. Whether you attached shelves permanently or create an adjustable system using predrilled holes and dowels, build the shelves first, then insert the box into the hole for the perfect place for all of your house décor.