The balls are placed in the each corner of the goal circle.
The goalkeeper runs out, picks up a ball, runs to the coach and hand it over.
Then quickly backwards into the goal to block the shot from the coach.
The shot can be directly or a lob can be performed.
After the shot the goalkeeper runs out and pick up another ball and so on.

Don't practice too long. Use 6-7 balls, take a break and then another 6-7 balls.