Sometimes I think the easiest way to train a behavior is to reward it when it occurs naturally. Wait for your dog to bark and be ready with lots of praise, the word you want to use for it, and treats if you use them. Be consistent and make a big deal of it.
Diesel is probably too young to catch on to this quickly but don't underestimate him. I think he's perfectly capable of forming repetitive associations in his mind over time and then one day, completely out of the blue, he'll have it
i taught my last GSD to "bark", "speak" on command.
if i remember correctly he was 1 plus years old before
i taught him to bark. i use to have him sit in front of me.
i would look at him and bark. then i would say "bark".
he eventually caught on. once he started bark on command
i started saying "speak".