Remove the window frame
The window itself must be left in place. The inner shells need to be padded, 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 bring any table decoration to its doom. Use a level to find the exact size of lumber to nail around the inside edges of the windows. Part of the casing may need to be removed, in other cases; wood should be added so that the inside frame of the window is flush and the measurements are consistent. Also remove any siding around the window wall.
isolation
Insulating the window and around the shelf box is crucial to ensuring your home stays comfortable and warm or cool and dry during the extreme weather months of the year. Fiberglass insulation works best, but ribbed foam panels work just as well, as long as they fit. Never use expanding spray foam as it can easily expand and break glass or push your shelving system off level, causing table decorations and other decorations to slide to the 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 shelf. Cut two boards the same length for the horizontal members. Now cut the two vertical members three inches long so that they all fit with the vertical member sandwiched in the middle of the horizontal members. Attach them all together. Cut a piece of 3/8-inch sheet metal to fit the back of the shelf. Use the factory edges of the sheet metal to ensure you get a straight cut. Now attach the shelf box to the veneer to create a perfectly square shelf box, almost ready for your wall art and décor.
Hardware and shelves
Place the box in the hole and check that it fits snugly, that it is level and flush with the outside of the walls. Shim as needed. Once you’re sure it’s a good fit, you can easily remove the shelf and add a shelving system as needed. Whether you’re fixing the shelves permanently or creating an adjustable system using pre-drilled holes and plugs, build the shelves first, then insert the box into the hole for the perfect spot for all your home décor.