Ok, the trick is a b/s crooked grind. Approach the rail or ledge at a comfortable speed to ollie at. When you get to the side of the rail (or ledge, I'll use the example of a rail), make sure you're facing away from the rail. Ollie up and into about a 45 degree f/s ollie. Push forward on your nose and land on the rail with your front truck and back truck diagonally behind you in the air. Use your back foot to guide where you turn at. Keep your balance while sliding. For the dismount, give a tiny nollie to get out. Practice landing, it's the hardest part. You want to come down and have your back foot push down to land flat.