How to do full body inflation?

That's because its only fantasy. The human body doesn't work that way. For it to work, your body would have to be just a bag of skin with all the muscles and fat and other tissues completely separated. It's not that way. Sadly, full-body inflation is just impossible.

Now, if it's the look you're going for, most people use an inflatable suit of some kind. You can find several threads covering those if you look around.

Sorry if that came off rude, but it's just fact. Hope it helped.