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.