The best way for a man to wear a scarf depends on the scarf and the rest of his outfit. A Parisian knot is a popular choice that works well with leather jackets, although it isn't a good choice for bulky scarves. The once-around knot looks good with blazers.
The simplest way to wear a scarf is draping it over the shoulders, which looks good with a suit or sports jacket. A twice-around knot works well with crew necks and short jackets. It can also be tucked into a jacket for extra warmth. A twice-around knot requires a longer scarf, ideally one that is at least 72 inches long.