literally just takes practice, balance and control. once you have them, you'll only make them carry on for longer. i can do them forever i just dunno how to describe learning them try stay low with your butt close benind your rear hub/axle, that's how i do it and its where the balance point is pretty much, then just slowly lean forwards or backwards if you upset your balance point.
if you try a manual normally, even if the front is tipping down loads, just keep leaning and pulling back, even if it the front doesn't come back up super quick, if you get it back up you know how to control them! its all about small, slow movements with your butt/knees whilst your in a manual. but i wouldn't bother moving your stem, kinda defeats the point of doing a proper manual... and looks weird! i reckon it'll be harder with the stem back too!
hope that makes sense... i dunno how to describe it very well
however, it makes perfect sense to me so if only i could show you face to face then you maybe, just maybe, might understand where im coming from haha
but if you do already then keeeewwwwlllll