Summarize the following:
To start, put on your shirt and button it. Grab the shirttails and pull them down towards the floor. This gathers any extra material at the bottom of the shirt and brings the material tight over your chest for a professional look. Finally, give yourself a  quick once-over examination. For a good tuck, the line formed by the buttoned-up edge of your shirt should match up with the line of the fly on your pants. This is called the "gig line" and, while it's not always something you'll want to spend the time and effort getting just right, for the most professional look possible, it's indispensable. Since your belt buckle should be in the center of your body, your gig line should intersect it or else lie quite close to it.

Summary:
Pull your shirt as far down as possible. Line up the buttons of your shirt with the fly of your pants.