How to Install Full-Frame Replacement Windows

Measure the width and height of the opening, with the old windows still in place. Measure from the furthest outward points, sliding the sashes up and down as needed to access the sides with the tape measure. Take several measurements in each direction to determine if there's any variation--for instance, if the opening is out of square. Take the shortest measurement you came up with for both the width and height, and subtract 1/2 inch from each. Use that measurement as the size for your replacement window.

Set the new window unit near the opening when it arrives. Remove the old windows, by first removing the wood trim that borders the inner edge of the window, using your hammer and pry bar. Remove it carefully, keeping it intact so you can reinstall it later.

Remove the window sashes, putting them inward into the room. You may have to pry out a second vertical strip of wood trim to get the top sash out.

Cut a strip of batt insulation the size of the window sill, using your razor knife. Lay the insulation on the sill. Set the new window unit on the sill (on top of the insulation), and push the top up and into the opening. Push insulation in around the sides as you push in the window. Push it until it's stopped by the exterior trim around the outside perimeter of the opening.

Lay a level along parts of the window, inserting shims along the bottom and sides as needed to get it level and square.

Drive long mounting screws (provided with your window) into the sides of the opening through the holes in the window unit, using your screw gun.

Reattach the trim that was around the interior perimeter of the window, using your hammer and trim nails. Caulk the seams between the new window and the trim, inside and out.