You'll want to order a new window two to four weeks before installation.
To order a new replacement double-hung window for an existing double-hung window, you need to obtain exact measurements from the old window. Old windows are often not level or plumb, so four different measurements will be necessary:
To determine the right width of the new sash, take measurements at the bottom, center and top of the window inside the frame; the narrowest measurement will determine the sash width needed for the new window.
Measure the height inside the window. That length will be the height measurement for your new window.
If you don't think you'll be able to remove the old window and install the new window in one day, be sure to have a piece of plywood on hand that you can use to cover the open window area after you've taken the old window out.
Remove the inside stop from the side jambs with a chisel and hammer. Then remove the bottom sash (the main part of the window).
Take out the center stop and then the top sash. Use a utility knife and the back of a hammer to tear out the weather stripping. If you're working with an older window, remove the old pulleys, pull out the old sash weights and cut the ropes.
Measure and cut the jamb liners to fit the window frame. The jamb liners provide the channel for the lower sash to be raised up and down.
Screw in the vinyl jamb liners brackets on both sides. Fit the jamb liners securely against the frame of the window and parallel to the lower sash filler blocks.
Snap in the jamb liners on both sides.
Install the lower sash by sliding the pivot pin into the groove of the jamb liner and snapping it into the sash balance on each side. Then lift the lower sash into place and test the window operation.