How does one “practice” for something like this?
Dunno. But that guy (or gal) was a pro. Well done, top to bottom.
You start out in a much less challenging environment, but the fatality rate of this is very high, so there are some things for which you just can’t practice, you either make it through or you don’t.
I have no experience with wingsuits, but I am “an aviator.” As someone else stated, there have been several dramatic fatal accidents in this sport. A flight with this small of a margin for error can only be successful with a very experienced pilot and lots of detailed planning. The basic premise is that you know what the glide rate/ratio is so after doing a detailed terrain study you can determine if you can make it work. It still requires a lot of skill (and courage) to perform and something like an unexpected gust of wind, or a bird strike could ruin your day.
Kidding aside -- jumpers have instrumentation that record rate of fall, speed, time, etc. The jumper knows his/her glide ratio ahead of time and plots it on a topo before jumping.
First jump are usually made from an airplane while one learns to control your suit (control surfaces) and parachute (brakes).
Common places to jump are known ahead of time and don't need calculation for every jump. Much like a golf.