Step 1:
Decide what size window box you need for your window. It should be approximately the length of the window but the depth and width is up to you. It is easiest if you choose your depth and width to be the same as the width of the piece of wood you will be using to make your box.

Step 2:
Choose what kind of wood you want to use and make sure you have enough to finish the project. A box 30 to 36 inches long would require a 10 foot long 1 x 6 or 1 x 8 board. 1 x 6 is recommended.

Step 3:
Cut your wood into the required pieces:

2 pieces the same size for the front and back, the length of these should be approximately the length of your window and the width should be the depth you want for your box.

2 pieces the same size for the sides, the size of these should reflect the width and depth you chose for your box.

1 piece for the bottom, this piece should be the same length as the front and back pieces and 2 inches wider than the two side pieces.

Step 4:
Pre-drill holes in the front and back pieces: 2 holes on each end. Do the same on the bottom piece but drill an additional 1 inch hole to allow for drainage.

Step 5:
Glue the front and back pieces to the side pieces.

Step 6:
Using the holes you pre-drilled as guides, screw the front and back pieces to the sides with 1 1/3" galvanized screws.

Step 7:
Glue the bottom piece to the bottom of your window box and screw that in place as well.

Step 8:
Sand and prime the window box. It is best to use an oil based primer.

Step 9:
Apply the final coats of paint to finish up.

Dress up your window box
You can use a bed molding or a chair rail to make your window box more decorative. Before you sand and prime, glue the molding onto the box where you want it and use small finishing nails to firmly hold it in place. Then continue with the sanding and priming. After you have painted your window box, use stencils to add a variety of colors.

Credit: Renovate Your World