Step 3: Starting the Frame
Create a rectangle by screwing another short piece of 2X4 between the long pieces at the other end, as shown. Make sure the corners are square.
At this point, it's a wise idea to put this rectangle into the window opening to make sure it fits. It should slide in and sit against the window framing. (If it's a little loose, you can use shims to help wedge it in place during installation.)