This is one of the most important things to study on our online game design course.
Aim to get your player to the positive satisfaction loop :)
If you've never played Tetris here's how the Core loop would go it.
What: What is this game, how do you play this. You could understand from a single picture that the single piece should be matched with the others.
How: Adding a little arrow to a picture could help you understand that you can rotate the pieces. Otherwise you would learn it via tutorial or reading about the controls.
Why: Tetris usually only has one reason: don't hit the top of the screen or you lose. This could be expanded with a storyline or a deeper reason.
Goal: Your goal is to rotate the pieces so that they fit the bottom row as well possible to create complete rows which are then removed. Your goal is to not hit the top of the screen.
Challenge: The challenge to the game comes from the different shapes of the pieces, that they drop down and that the speed increases as the game advances.
Reward & Satisfaction: You get reward whenever you can fit a piece nicely as this helps you in the game. When ever you complete a row as this is removed as a reward. At this point we could show some particles effect to the player to celebrate this. Player gets satisfaction when ever a row is removed.