Patience, patience and patience ... after all, think about it. Like bunnies, in the wild chinchillas are a prey species. If their feet come off the ground involuntarily, it's because someone is about to eat them. If you really want to be able to pick her up, you could try doing it every day, a little at a time, but that doesn't work with every animal. When you do pick her up, make sure you support her feet, particularly her hind feet, the whole time.
With my bunny, I made a conscious effort to make sure I picked her up and held her every single day when she was a baby, so she got used to it. But it only worked with me, because my husband didn't do it every day, throughout her life, when he picked her up, she would either struggle to escape, or "play dead" which nearly gave him a heart attack one day.
Again, it doesn't work with every bunny or with every chinchilla, so you just want to get yourself used to petting her on the ground/shelf, where she is most comfortable. Patience patience and repetition ...