All you need to build a cottage-style window box is a few feet of 6- or 8-inch-wide trim (choose straight baseboard or casing, rather than angled crown), one 2-by-2-foot sheet of ½
-inch exterior plywood, water-resistant wood glue, and a shallow plastic planter to cradle the soil and flowers. Because most old trim is covered in toxic lead paint, either remove the finish with a wet stripper or seal it under a clear top coat. This is an easy one-person project, but it's more fun with a helper.
For full step-by-step instructions, shopping list, and tools list, see How to Make a Window Box with Vintage Trim