If it's available, there will be a counter outside your terminal staffed by uniformed Skycaps, and there will probably be at least a short line. You show your ID and tell them your flight/destination, or show the printout of your e-ticket. They print your boarding pass and tag & take whatever luggage you're checking. Some airlines/airports charge for this - if that's the case, there should be a sign indicating the price. It's expected that passengers will tip a dollar or two per checked bag, whether curbside checkin has a cost or not.