Determine where to place the flower box so you can decide the dimensions accordingly. Measure the length of the window if hanging it there so it is equally long. Decide its length, depth and width. If placing it on the ground, opt for two medium sized boxes in place of a very long one, which is difficult to move when filled with soil. Measure the spot and note the dimensions down on paper.
Purchase boards of pressure treated wood from a hardware store. Have a professional cut them to size for you, or do it yourself with a saw. You can select pine, cedar, oak, teak, redwood or any other type of wood, depending on your budget and personal taste. Keep in mind that softwoods such as pine are soft and easy to work with, but tend to decay with time when placed outside, even if painted.
Your flower box will comprise of five boards- two long boards for the front and back, two smaller boards for the side pieces and a base as wide as the sides but as long as the front and back.
Drill drainage holes in the base piece, spaced 4 to 6 inches apart. Place the base piece on an even surface, and hold the front piece next to it. Make sure the lower edge of the front is flush against the long edge of the base piece before joining them together by drilling a row of 1-inch woodscrews into them, spaced 3 to 5 inches apart.
Hold the structure firmly in place as you place one of the side pieces against the short edge of the base. Align it so their tops are even, and join them together by drilling screws through the base and the front that penetrate the side and hold it firmly.
Repeat the procedure with the other short edge and the back piece.
Check the stability of the window box to make sure it is sturdy and all the joints are secure, and reinforce any joint by drilling woodscrews in place.
Sand the edges of your flower box to make them smooth, removing any dust with a hand broom. Paint it a bright color so it stands out, and allow it to dry thoroughly overnight.