Prepare the boom by cutting it to the proper length. Then,
Using a laundry marker, mark the end of the boom on opposite sides as shown (the one on the right is the one we are working on).
Tape the boom securely to a marking guide, positioning one of the end marks so that it almost touches the guide.
I used another piece of tubing as a guide, but one can use almost anything that is straight, even a piece of wood.
In the next step, you will be drawing a straight line down the length of the boom.