This project is just a laser cannon and single stationary, yet dancing alien. You move the cannon with your left and right arrow keys and can change it's speed using the up or down keys. The space bar fires the cannon. I've added a bunch of comments to help walk you through what each block is doing. So after clicking "See inside," start with the comment that begins "START READING HERE." Hint, it's in the top left of the Cannon sprite's code.
You can see a variation of the laser cannon and alien in the game Space Invaders here: https://scratch.mit.edu/projects/371616266/