1. With a staple gun, attach flawed magnolia leaves to the top edge of the window box so the box will be completely covered. Then staple a full column of whole leaves so the tips of the leaves come above the lip of the window box. Secure each leaf by stapling it twice at its bottom. Continue adding columns, working from top to bottom, right to left. Overlap each column so there are no empty spaces, and overlap each leaf in a column to cover up staples in the previous leaf. At the bottom, fold the last leaf of each column under, and staple to the bottom of box.
2. Fill the window box to the top with dry floral foam. Poke birch branches into the foam, packing them very close together. As you place branches into the foam, weave a strand of lights around the branch bases. The magnolia leaves that poke above the top of the box should conceal the lights. Use pruning shears to trim all branches to the same height, cutting the branches about as tall as the window box to maintain a pleasing scale. If desired, hot-glue clumps of moss around the base of the branches for added texture and to help cover the light cord.