What has worked for me is practice, practice, and more practice. Yes, sometimes really frustrating but, practice makes perfect as they say. No learning books or special classes required. just a good eye, having the neccesary tools, and be willing to be patient as you acquire more experience and develop the art of creating your own characters in manga style.
As for practice is concerned, do you see a drawing and wished you could draw that same? Well, that is exactly what I did when I started doing my own doodles. Have a reference picture and imitate it, but no tracing cause that is cheating. At the first try, you might not get a really good piece of art, but don't get discouraged. That is really how I learned.
Below you will see some of my work. Hope it helps! ^_^