I only learned to do it by watching, reading and lots of practice. Practice movement with pencil and paper, have a go on "small sandwiches" doing simple shapes. You will find it becomes easier the more you do it.
There are lots of videos on youtube which will help - have a look at Leah Day's free motion quilting blog
http://www.freemotionquilting.blogspot.com/
There are video clips on here too