The first thing to learn on how to board up windows on a brick house is that you need to start by counting your windows and then measure the openings. Count on boarding all windows, sky lights, doors and any roof vents or places where wind could cause damage if it got into your house. If this is because you are leaving your home for a time and do not want your windows broken, you do not have to be quite so fussy.
To measure each opening from the inside to the inside horizontally on the exterior trim and vertically starting at the sill go to the bottom of the top trim. Add eight inches to both width and height to make sure the panel will overlap framing.
The plywood needs to be at least 5/8 inches but preferably greater exterior grade. Plywood is usually sold in sheets four foot by eight foot. You might have to use more than one sheet for a large opening. One thing I always emphasize on how to board up windows on a brick house is that you need to wear goggles and gloves.
To make sure you get the right type of fasteners for brick ask for help at the lumberyard, hardware store, or home improvement center where you are buying your supplies unless you are familiar with fastening heavy plywood to brick. The plywood panels have to fit very tightly because if a strong wind gets under them they will be ripped off.
Secure the panels at each corner then install the remainder, space the fasteners six inches apart starting at the center for windows four feet or under and three inches for more than four feet.