Other than choosing us to do your windows, this is probably the most important thing you will do in the ordering of windows for your craft. Time and effort should be taken during this exercise as the windows will only fit as good as the patterns supplied to us.
Use a stiff paper or cardboard (if overseas) or 3mm ply / MDF if local. Make sure the edges of the patterns are straight and true. Cut the patterns so that they fit the hole with clearance allowed to all sides. The pattern should have 1.5 to 2mm clearance to all sides if the boat is ally, wood or fiberglass. If you are working with steel, allow 2 or even 3mm to all sides, to avoid contact between the ally frame the the steel. The part of the aluminium window that goes into the boat is what we call our 'Box Size' or sometimes in the trade its refered to as the spigot size of the window. This is the size that patterns should be made to.
Allow the clearance and mark all patterns with, "this is your exact Box Size" to indicate that clearances have been allowed. The patterns should also have as much information as possible, ie: whether the markings are on the inside or outside, Port or Starboard location, glass thickness & type, window type (fixed, slider etc etc). The more information the better and please don't forget to put your name on them, you'd be surprised how many patterns that turn up with no markings and people say, "but I only spoke to you the other day???" Hey, we talk to many people all day long and sometimes we forget who said what, mark the patterns clearly and all will go smoothly.