Order your new window. Since windows may need to be ordered, do not remove the old windows until you are ready to complete the job. Choosing a pre-hung window will make the job easier.
Do not rush when removing the old window. Take your time, as hurrying the job can cause damage. Begin by removing the removeable glass panels from the window frames and set aside.
Run a utility knife around the perimeter of the window frame, where the trim meets the wall. Do this on the inside and outside of the house. This will help separate the drywall and paint from the window frame. Remove the interior trim from around the window. Use a pry bar to do this, but place a wood block under the bar to protect the wall.
Remove sash weights by cutting the cord. Use the cord to lift the weights for removal. The voids can be stuffed with insulation. Remove the remainder of the exterior trim and molding. Use the pry bar.
Cut through any framing nails used to secure the window frame in place. Remove any shims and slide out the old window.
Read the installation instructions that come with your new window. You may need to install a j-channel to hold the trim. If this is the case, you will typically need to trim some of the siding.
Hold the new window molding around the outside opening and trace. You may need to cut away some of the siding to fit in the new window. Cut the siding along the line using a circular saw.
Measure and cut a strip of drip edge that will be fit on the top of the window, between the siding and building paper.
Look at your window opening and determine if it has a brick mold or nailing fin. If it is a nailing fin put a bead of continuous caulking on the back of the fin, and if it is a brick mold apply it around the opening.
Set the window in the opening. The nailing fin or brick mold should be snug against the sheathing.
Plum the window frame, use a level and shim if necessary. Check to see if the window is square by measuring the distance from the upper right corner of the window to the lower left corner, and the upper left corner to the lower right corner. If they are the same measurement, the window is square. If not, make additional adjustments.
Nail the windows and space the nails according to the manufacturers' instructions. Typically with nailing fins you will nail into the frame, and with brick mold you will use predrilled holes where nails are driven through the mold into the frame. A window with a nailing fin will usually have a molding that is placed over it.
Fill any gaps along the window with insulation and trim any shims so they are flush. Apply silicone caulk around the window unit. Paint when the caulk dries.