The most important part of vinyl replacement windows is critical, that you measuring your old window opening properly and accurately. Vinyl windows are custom built and can't be returned if they don't fit. Most are manufactured in 1/4" increments, and you'll want to get the closest fit possible.

Raise the lower sash and measure the jamb-to-jamb width in three places near the top of the window, at the middle and close to the bottom, measure from the head jamb to the sloped sill just past where it meets the inside window stool. Don't assume that all your windows are the same size.

Draw a rough sketch of your house and number each window, then measure each of them separately. When installing vinyl replacement windows, first remove the old sash. Double-hung window has two sets of removable stops. All have an inside stop that holds the lower inner sash in place and a blind stop that separates the two sashes. The outside stop is nailed to the edge of the sash and is not removable.

Take off the inside stops carefully so you can reuse them. Cut the sash ropes and remove the lower sash, then pry out the blind stop and remove it. Remove the upper sash. Set the vinyl replacement window in the opening. It should fit snugly between the stool on the inside and the outside stops of the old window on the outside. If the new window is narrower, you may need to nail a strip of wood to the jamb to fill the gap between the window and the outside stop. Some vinyl replacement windows have stop extrusions on the bottom that match the sloped sill of your old window, others are flat.

If the bottom of your new window is flat, you may be able to use a piece of filler or doorstop as a support for the front edge of the window.

Center the vinyl window in the opening. Use a carpenter's square to make sure the window is square, and then shim the sides of the window directly behind the predrilled screw holes in the jamb. Screw the vinyl replacement window in place through the side jambs, then replace the inside stop.

Go inside and check the vinyl window to make sure it operates properly. Then go back outside and run a narrow bead of caulking all around the window to seal the gap at the outside stop.