Alan at Woody’s Workshop just shared a process for squaring up material. The process is primarily designed to square a piece of stock ready for the cnc. However, it could be used for general machining as well.
Inspect the stock and pick the side that has the surface that is the flattest and most appropriate for the end result you are looking for. Place this surface against the rear vice jaw. This eventually be machined to become Side 4 but for the moment just skim the top surface which will now become Side 1.
Seven steps in total that are quite easy to follow.