The flag sign looks like a flag fluttering on top of a flagpole. Your non-dominant hand is flat, facing down, to signify the ground. You dominant hand waves back and forth, like a flag blowing in the window, while your forearm stands for the flagpole.
If your baby takes an interest in watching flags fluttering in the wind, you can introduce the sign. As they get older, you can start to introduce adjectives to describe the flags. For example, a red flag, three flags, or a big flag.