Measure your brass tubing to the desired length and mark it with your permanent marker.
Secure the brass tubing in a friction clamp. Ensure the clamp is tight enough to hold the tubing but loose enough not to dent it.
Put on your protective goggles and gloves. Secure the tube by placing one hand about 10 centimeters from the cutting point.
Activate the cold saw and lower it slowly onto the tubing. The saw will emit hot sparks, so do your best to avoid them. Push the saw into the pipe until the cut piece falls away.
Ensure the cold saw has stopped running before you place it onto a surface. Once it has cooled, replace its protective cover and store it in a dry place.