Learning Morse code is similar to learning a new language and it depends on the individual as well as to how fast he can grasp the code. If you want you can simply memorize the above given codes but there are certain methods which will help you
Farnsworth Method – this method makes use of delaying the intermittent time between letters and words so as to give more time for a person to think and get used to the code and the time is decreased slowly as learning curve is progressed.
Koch Method – this method uses the standard speed of transmission both for the actual letters/symbols as well as for the intermittent gaps but uses only two characters for a start
These methods were techniques developed to aid learning, and you can choose whatever you are comfortable with, and once you know how to read morse code and vice versa, your speed with increase with practice.
So next time to want to write a talk to someone without letting anyone understand, just go dah dit dit dah.