Hi JB and welcome. Typically that job is done with a panel bit which has a sharp point on it for plunging. It is quite commonly used in the RV business to cut out openings, going through the metal skin and the sheathing at the same time. The easiest template is to screw some boards to the SIP and follow around the inside with the router. You'd need to calculate the offset just as you would if you were using a skill saw to do it. One way to ensure the inside and outside are at least close is to drill through both sides at all 4 corners. If you are installing a window I'm assuming you still need a frame to set it into so you can do the minor leveling needed at that stage.