The secret to nailing manuals is balance. The key is to find a good positioning on your board that your comfortable with.
Begin by riding your board at a good speed; the faster you go the easier it is to do them. A good trick to do is to straighten your back while doing them and wave your arms at your side.
It is as easy as it sounds. Manuals are a great beginner maneuver you can do on yo
ur skateboard to get even more comfortable with riding on it. Once you nail them and have your balance straight one of the upcoming skateboard tricks I am going to teach will come even easier.
Remember to always PRACTICE and good luck with manual.