Hello,
This method of communicating is called Semaphore, and two coloured flags are used, and the Alphabet as well as numerals from 1 to 10 can be transmitted.
'A' is transmitted thus-: Face the person you are signalling to, and with your left flag pointing down between your legs, hold the right flag out to your right side at an angle that would look like 25 to the hour if you were holding the minute hand of a clock in your hand. and so on and so forth.
'B' is transmitted thus-: still facing the person you are signalling to, and your left hand, still pointing towards your groin, raise he right hand flag up a few degrees until it is out at what would appear as 15 minutes to the hour, at the reader looking at you....
'C' is the same as A & B except now the right hand is vertical, and the left flag is still in your groin area.
I hope that has got you started, Good Luck.....Tony M