We recommend the use of EPS Bucks (manufactured by FoxBlock) as they provide R-12 insulation value as well as provide multiple attachment points on the interior face (for inset windows) and on each edge (for surface mounted windows).
An excellent installation video by FoxBlock for surface mounted windows can be found here.
ZEEs are used to strengthen and ensure window bracing is at 90 degrees. For a printable PDF, click on the image.