1. First, choose the proper pants. Light-wash jeans are a little Weird Science, so go with dark denim or a pair of rugged khakis. They don't necessarily have to be slim, but they should definitely fit you well: Avoid relaxed-fit pants and baggy jeans. Finally, throw in a belt to add some visual balance to the finished look.
2. After you've donned your chosen outfit, fully tuck the t-shirt. Then grab ahold of one side, and pull the fabric until it starts to come untucked. In front, the untucked fabric should stop at your belt buckle, and while it can pass the center belt loop as it runs around your back, it shouldn't stray much farther.
3. Once your shirt is properly positioned, try your best not to futz with it. The key to this look is a measure of indifference, or what some might call sprezzatura. You shouldn't seem to care about the way your shirt is tucked, so don't draw attention to it. In trying moments, remember the acronym WWBD: What would Brando do?