So how do you practice wing-suiting
You need to be an experienced skydiver before any responsible DZ will let you jump from their planes with a wingsuit on. You will also have had the necessary coaching from a qualified instructor.
Of course that doesn't stop someone from going hop! from the top of a cliff on their homemade suit--I guess that's were Darwinism kicks in.
Edit to say the previous paragraph is in no way directed to those who appear on the video in question, as it happens I've met those guys and they are world class (they actually make a living out of that). Should have watched the vid before I opened my mouth 
Perhaps more relevant to this forum though, this other video from the same site (but ignore the comments on the page)