First you will need to create your Alien Sprite.
I have used Ladybug2 in the scratch sprite library.
Then add this code to the sprite.
Next, create a second sprite to be the missile.
I have used Ball in the scratch sprite library.
Then add this code to the sprite. (Make sure you add the code to your second sprite)
Just like in other tutorials, if you change the values in the blocks of code, you can change the speed and location of the sprites.