Glue the front and back panels along the right, left, and bottom sides. The top must remain open to provide a slot for slipping in drawings and artworks.
Try to apply glue as close to the edges as possible to ensure that there is enough room for drawings to be easily put inside the window.
Instead of a window that looks out, make one that is outside looking in. Replace curtains with window shutters that can be opened and closed. Create the shutters by gluing a pair of identical rectangular pieces of construction paper onto the sides of the window.
The window can also be given a more three-dimensional feel. For instance, you can make a flower box by cutting the bottom of a small box and gluing or stapling it onto the bottom of the window. Paint it with acrylic paint to blend it with the rest of the window.