STEP 1. Establish where the structural stud is below the window. All windows have structural framing around the windows. Drill a small diameter hole on each side of the window where the bracket is to be mounted to confirm a strong support member is located. The pilot holes need to be level with each other. Click here to download a diagram of a typical window framing.
STEP 2. Mount both brackets with the longer lag bolt provided. Confirm that the backs of the backets are fully supported against the wall and plumb.
STEP 3. Mount 2X facia (typically 2ea 2x6n or 1ea 2x12) to the front flanges of the brackets. It helps to temporarily clamp the 2X facia to the brackets. Confirm the facia is level then drill small pilot holes into the facia through the bracket as a guide. Then secure the facia with the shorter provided lag bolts.
Note: The most simple facia is cantilevered past the brackets a few inches (this will help conceal the brackets from the front).
A better look is to have the lumber with a 45 degree bevel and have the lumber cover the sides of the brackets as shown in the photos below.
To do this, you will need to drill 6-8 small holes in the side of the bracket and attach the 2X material with 1 1/4" drywall screws. Since both the front and the side lumber is attached to the same bracket the fit on the corner will stay tight. Use the bracket as a template to establish the cut angle. Set the front facia first making sure the side piece will fit snug where it should, then clamp to hold position, secure the front then the sides. If you've done a good job so far, the rest is easy.
STEP 4. Cut 2X material to fit between the brackets and rest them on the bottom flanges. If you have a big window (over 7 feet wide) the bottom could sag. You might have to put an intermediate set of brackets in the middle, back to back to remedy an extra long span between brackets.
STEP 5. Cut plywood to fit between the brackets to serve as a back and let it rest on the bottom flange and lean against the back flange.
STEP 6. Soil directly against the wood will degrade it very quickly, so a liner is needed to protect the wood. Most hardware/home centers sell rolls of 16" flashing. Install the flashing on the back and bottom first, then the front with the bottoms lapped (do not seal). The drainage will be channeled towards the front of the box vs. the back which could cause stains on the wall.
STEP 7. The back plywood edge can be covered with 1"x 2" counter flashing (roof edge metal). Place a screw low on the 2" side to hold in place. It's a good idea to put some liner material over the ends to protect the brackets.
To print a PDF version, 
STEP 3. 
STEP 5. 
