A single-button jacket is obvious: you button the lone button.
If you are wearing a two button-suit, you only button the top button.
If you are wearing a three-button suit, you can button the top two. However, the top button on some three-button jackets are stitched into the roll of the lapel (called a 3-roll-2 lapel). In that case, only button the middle button.
Never wear a jacket with 4 or more buttons, no matter how tall you are. You’ll look like a basketball player who can’t dress himself.
Generally, you should button your jacket whenever you are standing – they are designed and tailored to look best while they are buttoned. You should always unbutton your jacket while sitting.
The general rule is that you never button the bottom button. You can thank King Edward VII for that one:
“Historically, in the early years of the suit as everyday menswear, it appears there were no formal buttoning rules. Look to trade magazines and illustrations from the earlier part of the 20th century, and one sees jackets with between one and five buttons, each buttoned in a manner that suited the personality of the wearer or the cut of the garment.
But much of this changed with a king who was too fat to button his jacket. Or at least, that’s what legend says.
According to the lore of menswear, in the early 1900s King Edward VII started the trend of leaving the bottom button of a suit undone.
Apparently, he grew so rotund that he was unable to fasten the bottom button of his waistcoat and jacket. To not offend the king, those associated with him started doing the same. The custom then gradually spread the world round (as England was still largely an imperial power with great influence across the globe)…
Today’s suits are constructed in a manner where to ensure proper fit and drape of the jacket, one must generally leave the bottom button open. Whether this was started because of King Edward, or simply because of evolving fashion, it remains the rule today. (Source: The Art of Manliness)