Lobster or crayfish as a pet, is a very easy thing to do. There is not much that you need to do.
My lobster's name is Lobbie. All my students loved him but they always asked me silly question like "when are you gonna eat it?"
I replied, Lobbie is my pet... I won't eat it... For example, your dog is your pet, how about I also ask you, when are you gonna eat your dog?