Measure out the opening of the garage door according to the specifications on your garage door. Transfer the measurements to the outside of the brick wall using chalk. Snap a chalk line along the sides of the wall opening to make straight lines for cutting.
Drill corner points with a masonry bit through the wall. Snap a chalk line from the inside of the wall to mark the opening, using the drilled holes as reference points.
Put on the appropriate safety gear of goggles, ear protection and gloves. Place the masonry saw equipped with a diamond blade at the highest point on one side. Cut into the brick along the line, allowing gravity and a gentle sawing pressure to pull the saw down the line.
Cut the line on the second side of the opening using the same procedure. Do not put any pressure on the inner section of the opening.
Saw through the lines on the inside of the garage wall.
From the inside of the garage wall, use a block chisel to separate the top of the brick wall from the garage. Hammer the top bricks with a mashing hammer to knock the cut portion of the wall out of the opening.