To tuck in a shirt, tuck it inside unzipped pants, then smooth it down thoroughly before zipping the pants. This is a basic trouser tuck.
Let the fastened waistband of the pants hold the tucked-in shirt. If the pants are loose, use a snug belt to hold the shirt down. For looser outer layers, put on a T-shirt first, so the extra layer of fabric can help hold the outer shirt. A snug waistband or belt will still be needed, though, to hold down the tails of the tucked-in shirt. For a relaxed, but still tucked-in look, or if wearing an oversized shirt, pull the shirt out 1 or 2 inches before fastening the waistband.