If using a clicker, you would start by clicking any and all interest in the toy you are using to play the game with. If he's already holding the toy in his mouth, you click for holding the toy. Once he is holding the toy in his mouth firmly for several seconds, you start clicking for any and all motion towards you with the toy. This can literally be a glance in your direction, just click any interest in you while the toy is still in his mouth. Once he's got that down, start clicking for taking a step or two towards you with toy in mouth. Next you stop clicking for that amount and wait to click for 2-3 steps, then 4-5, and so on and so forth. Basically, you are breaking every part of the behavior down into its simplest actions, and rewarding each of those steps until he is confident chaining all of those behaviors into one fluid action.
It should be noted that you should start by simply placing the ball in front of you, maybe a foot or two away. Don't start by lobbing the ball across a field. Keep him close so you can properly interpret his actions, IE the moment he turns towards you, the moment he makes any sort of step towards you, etc.
GSDs- Mulder, Ridley, Vega
GSD on the half shell- Rigel