How do you practice lying down on a frog? Dance fundamentals. I'm the worst at frog plopping.

The main thing you need to do is to open your hips. You need to be able to easily open into a horizontal fork before you can do a good frog plop. Usually you need to get someone to help you press your hips. But if you're the only one, you might consider purchasing a splitter machine that can help you open up your crotch. You need to easily open your crotch more than 180 degrees in order to plie frog more standard.