Final answer:
For a square one length below the shoulders haircut, you begin cutting at the nape of the neck, setting the baseline for the haircut to maintain a square and even shape.
Step-by-step explanation:
When performing a square one length below the shoulders haircut, the correct place to begin cutting the hair is normally at the nape of the neck. This method involves sectioning the hair and starting the cut at the nape to create a strong, clean line at the perimeter or baseline that can be followed through to the rest of the haircut. Professionals often begin at this point as it sets the length for the entire haircut and ensures an even, balanced cut. Throughout the cutting process, it is important to maintain correct body posture and use precise sections to achieve a square and even result.