If you wantto practice ,you need to find some flat ground,preferably firm.As you dont tend to slide as much as die on soft lumpy ground .Try lentgh of polythene with water (soapy to get that slide feel.In Mills in Paris they play on the tarmac car park and it pissed down,people were sliding into snake50!! While you can superman anywhere ,it truly is only used in certain places ie God/insert into snake .Going into the back corner you can end up oversliding and getting lit up.The hip slide can be stopped with the other leg bringing you into a shooting position quicker .So while it is a useful skill to have in your locker it only has limited application.It will however get you onto vid,s since cameramen love it ,especially when you also get li up!