Beginner Exercise 1 - Quarter circle motions and using input display
Training mode offers a very helpful feature called input display under display settings. This is incredibly helpful to learn quarter circle motions and diagnose input errors. The aim for quarter circle motions is to start with your thumb on the down button and to roll it to the direction you want.
The game engine defines a quarter circle motion as down, down-forward and then forward. This is tough to get used to, follow the below process to make this motion second nature.
Jump into training mode with Goku and turn on input display in the display settings.
Practice the motion without pressing any attack buttons. With input display on, you should clearly see three arrows. Down, Down-forward and Forward (236 in proper notation).
If you only see Down, Down-forward then you need to make sure you are lifting your thumb up enough so that down is not held but forward is.
If you only see Down and then forward, try to consciously slow the motion down and make sure you press both down and forward at the same time in the middle.
Once you see these arrows correctly in sequence, practice it (on both sides) until it looks right every time.
Now it's time to include the "Special" button. Perform the quarter circle and try to press the special button as you lift your thumb off the down direction and to forward. You should fire off a Kamehameha.
If it still is not coming out, consult your input display once more, if you see Down, Down-forward, Special, Forward then you need to hit the button later. If you motion looks correct and is followed by the Special input then you must be pressing the button a bit too late. Keep going and keep adjusting your timing until your input looks correct and your special is coming out.
Once you can do this 10 times in a row, jump over your opponent so they are on the left side and repeat the previous steps until you can do it 10 times in a row. This may seem like overkill but the quarter circle motion is very important.