Take out the window you want to replace, its casing, inside sill and window trim; measure this opening. Use these measurements to purchase an appropriately sized garden window
Place a 2-by-4 piece of lumber, cut to measure, on the bottom sill of the window. Secure it with nails.
Nail the shim to top edge of the window frame. This will serve as a spacer.
Nail finish-trim strips to the perimeter of the window opening.
Place the garden window into the existing opening. Use notched pieces of lumber (lumber with a 1-inch V-shape cut out of one of the short ends) to temporarily hold the window in place. Set the lumber so it is slightly angled against your home. If the lumber does not seem secure, use two screws or nails to tack it onto the house.
Use a level to make sure the window is placed in the opening plumb and square. Do this from the inside of your home. Make sure you take into account the thickness of a continuous support and flashing when you check to see if the garden window is level.
Mark the siding of your home around garden window for cutting.
Remove the garden window from the opening in the house.
Use a circular saw and cut through the home's siding. This will expose the 2-by-4 framing around the window.
Place weatherproof flashing paper behind the wooden sheathing of the window.
Apply a large bead of caulk around the edge of the window opening to help waterproof the window.
Place the garden window back into the opening and secure it with rust-resistant screws through the flange and into the house.
Add more caulk around the perimeter of the window, on the outside of the home.
Trim away the excess flashing paper with the utility knife.
Nail wooden filler strips over the flashing paper to provide a nailing surface for the window's new trim.
Paint the trim and allow the paint to dry completely.
Place caulking around the perimeter of the trim, on the side that will face the home, and mount it around the window. Nail the trim into place with nails, and fill any nail holes and joint crevices with caulk.