Measure the width and height of your window with a measuring tape. Walk inside of the barn and determine the approximate location where the new window will be installed. Choose a location for the window that does not intersect with any existing studs. Add 2 inches to each the height and width of your window and use the new measurements to mark the outline of the window onto the wall. Hold the spirit level to the appropriate place and use its edge as a guide to create straight, even lines for the window outline.
Drill a 3/4-inch hole into each corner of the window site. Use the outline you made in Step 1 to locate the exact location of each corner. Drill completely through the wall, until you can see outside through the hole. It is important that the holes go entirely through the wall, because you will be using these holes to signify the location of the window frame from outside of the barn. When all four holes are drilled, go the exterior of the wall.
Locate the four holes on the outside of the wall. Setup a ladder if the window is located higher than you are able to comfortably reach. Use the spirit level as a guide to connect the four holes with a pencil and mark the outline of the window opening. Use the pry bar to remove any nails that may intersect the cut lines.
Set your saw to a cut setting that is 1/2 of an inch wider than the thickness of the wall. This will vary from barn to barn. Cut out the window frame opening, following the outlines exactly. Pry the cut section of wall out with a pry bar. Use caution so the section does not cause injury or property damage if it falls to the ground. If possible, remove the section with your hands.
Build the frame of your window by cutting two pieces of 1-by-4-inch lumber to the exact length of the width of the opening that you cut into the wall. Cut an additional two pieces of 1-by-4-inch lumber to the exact length of the height of your window. Nail the top and bottom 1-by-4 sills to their respective edge of the window opening in the wall. Nail the two side pieces to finish the frame. The frame should be nailed to the opening so the frame has a depth of 4 inches and is 1 inch thick.
Install a window into the frame or leave the window opening empty, depending on the use and the style that you are trying to accomplish.