How does one make a horse that is inverted / u-shaped/ has upside-down neck?

Moonshine came to use super inverted, and even with all the bodywork and my daughter trying to ride her correctly, she never really got over that. Even in the pasture she'd travel inverted sometimes. In her case, we believe it was due to being ridden in a western saddle that bridged by a person or people who were too heavy for her, at the sitting trot, a lot.

I unfortunately don't think I'm a good enough to ride Rowan beautifully into true collection, but I do think I'm good enough to avoid making him inverted, at least as long as I know what to avoid.

So how do you ride a horse and make it inverted? Or, more pertinently, how can I ride him so as NOT to make him inverted?

PS I ride Pony all the time and he's fine, but he has a short back and a natural way of going that's not at all collected but not at all inverted either. Rowan has a longer back.