It may sound like a pretty straightforward task, but don't be fooled; there's a science to a perfectly tucked-in shirt.
First off, make sure that your shirt is suitable for being tucked in. It should have a full-length hem, with enough extra fabric in the front and rear to stay comfortably in your trousers.
1. Unbutton the trousers in the front. Tuck in the shirttails first.
2. Now tuck in the hem in the front. If your pants have an interior tab (this is more common with dress pants), make sure to button it. This will help keep the shirt tucked.
3. Continue by buttoning the front of your trousers.
4. You may have extra fabric billowing out above your trousers at this point. Pinch the side seams in for added slimness and re-tuck the shirt on both sides. If this doesn’t do the trick, consider wearing a slimmer-cut shirt.
If your shirt has a flat hem or a hem that does not obviously extend below your hips, it will not work well if tucked.
The answer here is simple: leave it untucked.