Does your dog know how to lie down? If so, then take your treat and lure his nose ACROSS his front leg and AROUND his shoulder. Most people just bring the treat to the floor for a play dead and that causes him to get up or squirm to get the treat. When you lure him across and around, he'll likely follow (it will take a few times til he figures out what you want. Keep your treat close to his nose and keep at it). Once he gets on his side, then put the treat on the floor so his face is flat on the floor. THEN give him the treat.
After a couple of tries, it will be one sweeping move. If he lifts his head, don't treat. His nose has to be flat on the floor. Once you have this, depending how advanced your dog's thinking and obedience skills are, you can start him off at a sit and give him the play dead command. You may need to lure him into the down and then through the play dead movement a few times so that he understands you want him to do it all together. A lot of people start off giving "Bam!" as the command, from the very beginning. This way, as you advance, you can then pair it with it the "gun" hand signal, which a lot of people find cute.
To roll over, once your dog is in the play dead position, you just lure him to roll to the other side. Be sure you're working on carpet or a rug for this. Some GSDs have bony backs and have a hard time rolling at first, so if they're trying to roll to follow the lure but are having a hard time, you may need to push them along and over. One of my dogs -- a large male -- couldn't tweak his back quite right to get the rolling action, so he used to grab on to my hand and push off on that with his paws until he developed the ability to do it on his own. So you'll want to be flexible with your dog's physiology.
Puppies can a lot more flexible of course,both in their bodies and how fast they grasp a new behavior. My 3 month old pup already knows play dead (we call it "take a nap" and roll over. I taught these in one evening, and after sleeping on it, he mastered them the next day.)
One more thing, most dogs naturally roll one way or another better (that includes on the play dead too). If you're not getting anywhere trying to roll your dog to one side, try the other. Once you find one direction that works, stick with that for a while though. Let him learn that thoroughly before you change directions on him.
With regard to shake, some often lift a paw (often it's a sign of deference), especially if they want something. If yours does, click and/or mark it with the word "shake" and reward. If yours doesn't you can hold a treat at his nose and lift it higher. At some point, one of his paws will usually go into the air. Touch the paw and mark it with the word "shake" and a reward.
If neither of these work, you can just lift your dog's paw into the air with your hand, tell him "good shake" and reward. It's easier and often faster to teach shake when your dog is already naturally offering the behavior, but I had a dog that didn't offer her paw, and I was able to teach her to shake by just taking her paw and rewarding her for it.
Have a great time! Tricks are great because they make learning fun!