Stand upright on the balance board. Put your feet in shoulder-width stance and try to balance yourself.
Strengthen your core muscles to balance yourself in an upright position on the balance board. Start by making several swings to the sides, i.e. by shifting your weight from one side to another. Then balance again and slowly lift up your heels in a squat position to stand on your tiptoes. Repeat this movement by alternating legs while keeping your centre of gravity in the centre of the balance board.
Make sure you keep your balance and keep your torso straight.
Keep your feet at an equal distance from the centre of the balance board.
Prefer abdominal breathing.
Start by holding onto wall bars or something similar. As you gain more balance you can gradually do this exercise without assistance.
Balance board is designed in the first place to achieve balance, not to strengthen the calves. It is, however, a very effective calf exercise. If several types of balance boards are available you can start with the easier ones to learn the proper movement.