With constructions

ball.y = ball.y + 8;

But this is not correct. The right side of the assignment gets the value of the y-coordinate of the first ball and adds 8 to it. Next this new value is set as y-coordinate of all balls. So the result is that all balls get the same y-coordinate. The statement

ball.y += 8;

will have exactly the same effect because it is simply an abbreviation of the first statement. So how do we achieve this? For this purpose there is the with statement. Its global form is

with (<expression>) <statement>

<expression> indicates one or more instances. For this you can use an instance id, the name of an object (to indicate all instances of this object) or one of the special objects (all, self, other, noone). <statement> is now executed for each of the indicated instances, as if that instance is the current (self) instance. So, to move all balls 8 pixels down, you can type.

with (ball) y += 8;

If you want to execute multiple statements, put curly brackets around them. So for example, to move all balls to a random position, you can use

with (ball)
{
  x = random(room_width);
  y = random(room_height);
}

Note that, within the statement(s), the indicated instance has become the self instance. Within the statements the original self instance has become the other instance. So for example, to move all balls to the position of the current instance, you can type

with (ball)
{
  x = other.x;
  y = other.y;
}

Use of the with statement is extremely powerful. Let me give a few more examples. To destroy all balls you type

with (ball) instance_destroy();

If a bomb explodes and you want to destroy all instances close by you can use

with (all)
{
  if (distance_to_object(other) < 50) instance_destroy();
}