When the measurements are done it is time to start cutting in with the Xacto knife.I started with what I call the dining room window which is the largest on the back facade.
Keeping your xacto (new blade,change often) 90 degs to the surface cut into the foam board.I do this freehand and make a couple of passes until it is cut all the way through.At this stage I don't try to be too exact as the foamboard is soft and easily damaged.Put another piece of foam board behind the cutout and trace the perimeter of the window.Cut this piece out as well and line it up and glue it to the back creating a double thickness.This will allow for a secure surface for our window frame pieces and add a more realistic look to the thickness of the wall.When dry start fitting your frame pieces,do not glue for now but make a tight fit.I cut mine slightly oversize and fit and sand,fit and sand until they fit snugly .Get out a small square and adjust the foamboard until the frames fit 90deg to each other.
It is time consuming to do but I thank my lucky stars that I am only doing a plain outdoor facade and not your typical Victorian moldings that you would find on the inside.
