we train "snort on command" right now, we're working on similiar stuff.
With the "bark on command" I just barked at him until he'd start barking back and then mark and reward. After he understood that he'd get a reward for barking after command, I accompanied it with a handsign and then I changed the barking sound to the keyword.
probably not the most effective, but it worked for us.^^"
I never could teach Chess to bark on command, because almost nothing makes her bark. The only time she does is when someone goes past the yard and she goes off at them. And obviously I can't reward or catch that.
But with Echo, she's easy to make bark, so I just barked at her and rewarded when she barked back. It was one of the easiest things I taught her. If you know any way to make your dog bark, just do that and reward it.