To install a new window on an existing frame, clean the window frame, take out the existing inner stops, sashes, caulk and nails, and ensure that the replacement window fits the frame correctly. To install it, caulk the sill and outer stops, position the window appropriately, and replace the inner stops. Cover any gaps between the window and the frame, and apply caulk on the seams.
To ensure that the replacement window is of the right fit, determine the window's width and height accurately. To do this, measure from one side to the other and from the top to the sill at three places, and note the smallest measurement.
Begin installing the replacement window on the existing frame by cleaning the sill of the window. Ensure that it not rotten using an ice pick or a chisel. After removing the window sashes and inner stops, clean the frame. Then, take out the existing caulk and nails.
Next, apply caulk on the inner edge of the outer stops and the sill of the window. When positioning the new window on the frame, ensure that it is placed square and plumb. Check the plumb with a level, and measure the distance between the diagonally opposite corners to keep the window square. Use shims to hold the window in its position.
When replacing the inner stops, use fresh nails that are 1 1/2 inches long, and apply caulk on the joints between the stops and the frame of the window.
Insert the sloping frame available with the window from the outside to seal the gap. Alternatively, use a wood piece for this purpose. Apply caulk on the seams between the cover and the window frame to seal the joint.