Lay the scarf flat on a table. Fold corner to corner to form a triangle.
Drape the triangle over your head so that the point C is in the back and points A & B meet centered below the chest. Put a hijab pin (black oval in pic) just underneath the chin to hold the scarf in place.
Optional: One popular way is to take points A & B cross them over one another and pull them back behind the head, then tie them together loosely behind the neck. This helps the scarf stay on during prayer when prostrating (the sujud).
Be creative: The directions above are only a guideline for wearing a square scarf the classic way. Scarves can be folded in many creative fashionable ways and still cover properly as a hijab.
An alternative way to wear the scarf is to drape point A and pin it on the scarf at side of the head with a fancy hijab pin and/or drape point B across the chest and pin it to the shoulder with a brooch.