Final answer:
To begin a triangular one length haircut above the shoulders, start with a central parting from the front to the nape and first cut the nape area, before moving on to the sides, using consistent tension and angles.
Step-by-step explanation:
When performing a triangular one length haircut above the shoulders, you typically start with a central parting from the front to the nape of the neck. Then, begin cutting at the nape area, sectioning off small parts of the hair and cutting it to the desired length. It is important to use consistent tension and cutting angles to ensure that the haircut is even all around. Once the back section is cut to the desired length, continue with the side sections, using the back as a guide to maintain the triangular shape and evenness.