The key to a pull over is both arm and core strength. I suggest doing tons of pull ups, push ups, and dips to increase arm strength. And a lot of tuck holds, L holds, sit ups, hollow rocks, etc. to work the core. Once you get the core and arm strength up, the pull over will be a piece of cake!
Just remember that on a pull over, you must pull your chin up to the bar first before moving your hips over the bar. And keep your head up! If you drop your head, it makes it harder to bring your hips all the way around the bar. Good luck!