The easiest way is:
Decide which way you're going to 270 off before you hit the rail. If frontside, get on the rail, but don't turn your body very much, so you already have your upper body starting the rotation by the end of the rail. If backside, turn your body past your legs when you land, so you can lead with it. When getting to the end of the rail, you can either lean to the side of the rail slightly for some leverage to jump off and spin, or just pop straight up with your knees. Like extend your legs quickly, but not very much. If you do it too much, you'll just bobble the whole thing.  Make sure your body is setting the rotation AFTER you pop.