If something is done properly, it is done in a correct and satisfactory way.
You're too thin. You're not eating properly.
你太瘦了。你没有好好吃饭。
If someone behaves properly, they behave in a way that is considered acceptable and not rude.
He's a spoiled brat and it's about time he learned to behave properly.
他是个被宠坏的顽劣孩子,该是他学着行为得当的时候了。