German Shepherds enjoy having a job to do, and need frequent mental stimulation. Teaching your German Shepherd tricks is a great way to satisfy his needs in those areas. Because your dog is probably very intelligent and desires to work for you, he can excel at all different types of tricks. Many tricks build on one another, and the more tricks that you teach to your dog, the easier it will be to teach him other tricks that require similar actions. By teaching your dog how to 'shake', you are also teaching your dog how to lift up his paw on cue, and how to place his paw on something. Those skills can help you teach other, more advanced tricks, such as 'limp' and 'contact'.
'Shake' is a great beginner trick to teach your German Shepherd. Because there are multiple ways to teach this trick, and because all your dog really needs to know before you can teach this trick is how to 'sit' and how to tolerate a moderate amount of touching, most dogs can easily learn this trick.
Expect this trick to take between two and four weeks to train. Of the three methods, the 'Hold' method is probably the simplest method for you to use, but if you choose to use that method, expect it to take your Shepherd slightly longer to learn 'shake'. This is because methods that require your dog to initiate the behavior himself tend to be learned fastest, and with the 'Hold' method, you will be doing most of the work for him at first.
If your German Shepherd is not used to having his feet touched or is afraid of having his nails cut, then you will need to spend time teaching your dog to like having his feet touched at first. To do this, gently touch your dog's foot and offer him a treat. Repeat doing this until your dog enjoys having his feet touched. Only touch your dog's feet if he is not likely to bite you for doing so though.