Measure the window opening height from top to bottom of the casing, minus 1/4 inch. The recorded width should be the measured width from side to side of the casing, minus 1/4 inch. This will give 1/8 inch clearance all the way around.
Work left to right, and floor by floor, recording each windows measurement.
Give each window a designated name or number for future reference.
Measure the window's opening, consistently measure height, then width of each window. Sticking to this method helps keep you from making careless measuring mistakes.
Add 5 inches to each dimension. So, if a window measures 24 inches wide, Record it as 29 inches wide. This gives you around 2 1/2 inches of plywood on each side past the window opening.
Work left to right, and floor by floor, recording each windows measurement.
Give each window a designated name or number for future reference.