Measure the width and height of the door unit using a tape measure. Add to the measurement if you're using studs and headers to install the door. Use a felt tip marker and a chalk box to transfer the width and height measurements to the basement foundation where the door will be installed. Check the marks with a 4-foot level to ensure the lines are plumb and level.
Hold a 16-inch-long two-by-four block of wood vertical, 4 to 5 feet up and centered in the doorway that will be cut out. Drill two or three holes through the wood and into the foundation material with a hammer drill and a 1/4-inch-diameter masonry bit. Slide three 16-penny nails into each hole and hammer them to secure the block of wood to the foundation.
Wedge a piece of lumber at a 45-degree angle, or close to it, between the floor and the block of wood. Repeat the wood block and lumber installation on the opposite side of the door portion that is being removed so it won't fall once the concrete saw cuts have been made.
Inspect the concrete saw for any defects or worn parts. Top up the saw's gas and oil levels and install the blade that you will use to cut the foundation. Turn on the water source if you're using a wet saw and start the concrete saw following the manufacturer's directions.
Cut a horizontal line as close to the floor slab as possible, followed by the vertical lines that were snapped with chalk, working from the bottom up. Keep both hands on the saw at all times and take breaks as needed to avoid fatigue. Cut the top horizontal line last. If the cuts didn't go completely through the wall on the first cuts, repeat the steps on the opposite side of the wall so the cuts meet up.
Remove the wedged board in the direction you want the cut slab to fall, making sure no one is in its path. Cut the portion into smaller pieces for easy removal. Remove any remaining tabs of concrete around the doorway using a masonry chisel and mallet. Install the door.