Heisoku-dachi (literally: “closed-feet stance”), the attention stance is analogous to the attention posture used by military units around the world. This is the position one assumes during formalities, such as bowing.
Stand upright, with your back ramrod straight, keeping the chin up, chest out, shoulders back, and stomach in. Your arms are perfectly straight, and fixed at the side. Both feet point directly forward, with both heels touching, and both big toes touching. The head and eyes look forward, without looking around. No smiling or goofing-around; act serious, act professional.
[Photo of attention stance from the front and side]