To measure neck size accurately, follow these steps:

1. Wear a collarless t-shirt to ensure nothing adds bulk around your neck.
2. Stand up straight with shoulders relaxed and your head in a neutral position.
3. Use a flexible measuring tape and wrap it around the widest part of your neck, typically just below the Adam's apple.
4. Ensure the tape is level all around, snug but not tight, resting lightly on the skin.
5. Read the measurement where the tape meets. For dress shirt sizing, add 0.5 inches (1.27 cm) to this number.
6. Repeat the measurement a few times for accuracy.

Tips include measuring at a consistent posture without tilting your head and making sure the tape is horizontal and not twisted or angled.