To install windows to an existing home, you need to first measure the size of the existing window so that it can be the same size with the new one. Remove the old window easily and clean up all edges so as to make installation simple. Take the measurements needed from the old window so that you can be able to adjust the new measurements as required. Nail into place the new window and then use a window tape to cover all seams. See more details at http://robjundt.hubpages.com/hub/How-To-Install-New-Construction-Windows-In-an-existing-home.