I do uthrow to fair multiple times a match, I basically just mash jump as the animation finishes, and throw a fair. I've had trouble doing it consistently without double jumping - even using the air jump as soon as Bowser is off the ground, he accelerates up faster with the air jump than the ground one. And you want to be getting up there as fast as possible. With the way some people DI on for glory it looks like a guaranteed combo sometimes. Depending on DI you may want to go for a uair instead, by the way.