The flowrider has a metal bar or Rail, at the top edge of the flow surface. In a rail slide, the rider balances their flowboard on this bar with simultaneously carving accross the wave.
1 - Get really good at carving before you attempt a rail slide.
2 - Practice balancing yourself on a board on top of a 3-inch PVC tube
3 - If you want more practice, and have a death wish, set a PVC tube on a staircase and slide down it while balanced on your board.
4 - Put it all together on the wave and slide like a champ!