As you approach the rail put your feet together so it is easier to keep them together during the vault
Put your hands shoulder width apart on the rail and jump up almost verticly.
Bring your knees up into your chest to avoid trailing legs from hooking on the rail
Push on the rail with your hands to gain a little bit more height just to ensure that the move is a success
Once you have overcome the rail throw your hands forward to gain more distance
Land it how you will but i thought it would be funny to show every one Rob's gymnastic land