Mounting a window flower box may seem like a task for only the super-handy man, but it isn't as difficult as it seems. Even if you're a complete novice at using a drill or really installing anything, you can mount a window box yourself. When you purchase a window flower box, it should come with two mounting brackets. Many even come with the screws you'll need to mount the box to your house. One of the most important things you can do to mount a great-looking window flower box is to measure and determine the right size.
Measure the window, not including the trim. Purchase or build a flower box that is 3 to 6 inches longer than the window.
Mark below the window on either side of the window to plan where you'll install the brackets. Then, measure and mark 1 inch below the bottom windowsill if your windows are sliding or double-hung, or 6 to 8 inches below the windowsill if your windows swing outward when you open them.
Line up the first bracket on the left side of the window, placing it over your marks. Drill a "pilot" hole for the top screw into the bracket using a countersink drill bit. If you have aluminum or vinyl siding, drill a pilot hole into the siding.
Screw the bracket to the siding through the pilot hole using the screws that were provided with the brackets when you purchased your window flower box. If your window box didn't come with the screws, you can use 3-inch galvanized decking screws.
Repeat this process for the second and third screws in the bracket. Hold a carpenter's level next to the bracket while you're installing the two screws to ensure that the bracket is straight.
Line up the right-hand bracket with your pencil marks. Set a carpenter's level across the two brackets to ensure that both are level with each other. Repeat the entire process that you did for the first bracket.
Place the window flower box on the brackets with the flat side facing the house. Measure the overhanging edges of the flower box on the bottom to center it on the brackets.
Drill two pilot holes into the flower box's back side, directly over the brackets and toward the top edge. Screw into the holes the 2 ½-inch decking screws or the screws provided in your window box assembly kit to secure the window box to the brackets.