Words of wisdom by Bas Koole (http://www.baskoole.com):

The 313 is basically a Railey with a backside 360. After you take off for your Railey, you turn your body in the direction of “wrapped”. If you will not pass in the air you would land heelside, with your bar behind your back, what is called “wrapped”.

It is really important that you can do a steady and powered Railey. This will give you the ability to start turning in the air. Make sure you edge hard so you can take off for a powered Railey. As soon as you are in the air keep your body stiff and don’t swing your legs up for a mega over stretched Railey, you need to keep your body in control. You can do this by activating all your muscles, specially your stomach and arms.

Now you are in the air and ready to make the rotation. Pull yourself into a straight position, so you can make a proper flat 360. It could help to put your shoulders back and bring your chest into the rotation.

Steer the kite down a bit with your front hand, this will make it easier to pass the bar. While you are steering the kite push the bar down to your hip. Make the rotation and pass the bar behind your back.

As soon as you passed the bar, try to get your backhand on the bar again. In the beginning this will not be easy, but the more you get into the trick the easier it gets.

Extra:
After a while you can start to try tweaking the board, waiting longer with passing the bar or you can start adding grabs before and after the pass. Give it your own signature.