oh that, it's the same way you do it with every character. there's no real specific timing for any b-reversed move because the input is all the same for the majority of the characters. there are just some moves that really can't be b reversed or have little to no distance changed when performed.
anyway, jump with bowser, and say for example you're jumping to the right.
what you wanna do is flick the joystick to the left. what this does is make it so that if you were to only press b this time, bowser would shoot fire, but to the left--bowser would still be going towards the right though.
you know this though, the thing you want is this: you do basically what i said above, flick the stick to the right, and then b-reverse in the direction you've been facing to begin with.
it should look like this: [jump/be in the air], hold -> (sometimes you don't even have to do this), flick <- (or opposite direction), b reverse in the direction you were facing to start (which in the example's case is ->)