What is involved in installing replacement windows?

Let's assume you have the old windows out. Make sure you buy replacement windows and not ones intended for new construction. You need to install a sized window, level it and shim the sides, not too tight. You will need to make/buy extension jams to match your interior wall surfaces as well as re-install the interior trim. The exterior should be covered with a 6" rubber membrane that adheres to the window frame and under the siding as much as you can reach. Although the membrabe has a strong adhesive it will also get aditional attachedment when you re-install the brick molding and sill or whatever is outside. Before re-attaching the interior trim I'd add some insulation into any cavities you see around the edges, don't pack too tight, fluff not stuff. Do not use expanding spray foam for windows or doors.