The first thing to do is to use a prybar to remove the window trim and jamb extension from around the old window. The jam extension is just a piece of wood that’s been ripped down and attached to the window jamb to bring it even with the wall.
Unscrew the window from the jamb and remove it. Take care not to break the glass while removing. Many windows will be multiple window sashes with moving tracks on the side. Those types of windows may be easier to remove one sash at a time.
With the window out of the wall, inspect the condition of the window jamb to make sure there is no structural damage. It’s not uncommon for an old window to leak, rotting out the jamb. If there is damage, you will have to replace or repair the damaged areas of the jamb before moving forward. Also, you will need to measure your rough opening to make sure that it can accommodate your new window. The rough opening should be 1/4” to 3/8” larger than the exterior dimensions of the new window.
The window stop is a piece of wood ripped down and placed around the exterior side of the window jamb. The function of the stop is to prevent the window from sliding out of the opening. It literally stops the window from moving. It’s very likely that the original stop is still in useable condition, but if it is damaged, you may need to repair or replace this as well.
Before applying the silicone to the the window stop, dry fit the window to make sure there are no fitment issues. If you do have fitment issues, address them now and then dry fit the window again. When done properly the window should fit snugly into the opening. Do not move forward with the install until you are satisfied with the way the window fits in the opening.
The actual install of the window is very simple. First, run a bead of silicone on the inside edge of the window stop (Image 1) and then slide the window into place (Image 2). Place a level across the top of the window and press cedar shims between the bottom of the window frame and the jamb as necessary to get the window sitting level (Image 3). The window will usually come with its own hardware packet, typically consisting of four screws that will go through the sides of the unit to anchor it to the jamb. Two screws should be put into each side of the window, one near the top and the other near the bottom (Image 4). There will be pre-drilled holes in the window frame for the screws.
There may be gaps around the original window frame the wall studs, and this is the perfect opportunity to insulate those areas better. For larger gaps, cut pieces of roll insulation and pack them into the gaps. For harder to reach spots, you can use expanding spray insulation foam.
Pick a style of molding that fits your home’s decor and create a frame around the window. Measure the length of each piece and use a miter saw to cut the trim to the appropriate length. Make your cuts on 45-degree angles so the trim will fit together with a nice picture-frame look. Attach the trim to the wall with finish nails. You may also need to add thin strips of wood trim (a jamb extension, see Step 1) inside the original window frame to cover the gap between the new window and the drywall.
At first, use only enough nails to hold the trim to the wall until you’ve successfully lined up all four sides of the frame. Once you’ve achieved the proper fit and you’re sure the trim will not require additional adjustments to fit, you can go back and add more finish nails, every 6 to 8 inches. Any slight gaps in the corners can be filled in with silicone before painting.
The final step is to finish off the window framing by filling any nail holes with stainable/paintable wood putty and painting or staining the trim moulding around the window for a finished look.