After a 50-50 (riding along the rail straight), this is pretty much the easiest jib trick you can try. It’s called backside cos you approach with the rail behind you, not cos of the way you’re facing. There’s a more detailed explanation of how to do it here, but the basics are:
1) Approach with the rail on your heelside.
2) Pop onto the rail and turn your board 90 degrees. Try to keep your board totally flat against the rail and not to lean too far forward or back. Looking at the end of teh rail can help.
3) Slide off the end of the rail and pop your board back around 90 degrees to ride away. Or pop round to switch to mix it up if you’re feeling really revolutionary.