Choose a window design very similar to your current window, or you will have to make structural changes to your window openings. This article does not cover structural changes.
Examine the window sill on the inside and outside to ensure that the window frame is sturdy. There should be no rotten or soft spots in the wood frame.
Measure your window. There are several measurements that you will need. Measure from side to side on your window at the top, bottom, and middle. Then measure your window from top to bottom at the left, right, and center. Use these measurements to order your replacement window from your local building supply store.
Remove the stops on the inside of your original window frame. Keep them for use with your replacement window. Take the original window out of the frame.
Remove the sashes from the frame. Using a pry bar, pull the sash away from the frame, and then use a hammer claw to remove the nails.
Scrape the old caulk off the frame with a paint scraper. Clean away all of the debris.
Hammer any remaining nails down flush with the frame or pry them out.
Place your replacement window into the empty frame to make sure that the window fits properly. Remove the replacement window from the frame.
Insert the tube of caulk into the caulking gun with the tube nozzle pointing away from the trigger. Pull the trigger on the caulk gun to put a thin bead of caulk on the window sill and the inside edges of the outer window stops.
Place the window in the center of the window frame.
Use your level to ensure that the window is level and plumb.
Measure on the diagonal from corner to corner to make sure that the window is square in the frame.
Place the shims at the bottom and sides of the window to hold it in its proper place while you work.
Replace the inside stops in your window frame. Use the 1 ½ inch finishing nails to install the stops. Leave the nails out about ¼ inch. When you hammer the nails into the inside stops, the nails act together with the caulking to hold the window in place.
Set the nails using a nail setter to avoid damaging the window.
Caulk along the edges of the window where the inside stops meet the window.
Go outside. Caulk along the seam where the window matches the sill.