Okay. The only thing that you really need to understand is what all of the numbers do in the "set position" block. Let's review that.
The first number is how wide you would like your oval to be. For example, if you want a tall, skinny oval, then make this first number a small one. If you want a short, fat oval, make this number a large one.
Don't worry about what the variable does, just put it in.
The third number is the position of the center of the circle. If I want my circle to be in the center of the screen, then I set this number to 500, because the center of the X axis is 500. Tell me if you understand this part.