As said above, making sure to be there to aid your mantis during her molt is key.
Just in case you miss it, though, keep the bottom of her enclosure soft and limit the amount of objects in her habitat she can bump into. I’ve had a mantis fall during a molt, and normally, as long as they’re not hurt too badly in the fall, you’re able to help them out. I’ve also witnessed another one of my Mantids molt standing on the ground. Still don’t know why he did that, but he came out fine.
Also, make sure that if you have to pick her up if she’s fallen during the molt that you grab her exuviae, and not her body.