practice drawing 3 dimensional shapes like spheres and cylinders and cubes. Then shade those. Its easier to shade those shapes because you know which areas to designate to be darker or lighter.
After that you apply the same rules to your B shape or Letter shaped 3 dimensional objects. But in order to apply those same rules you need to understand how to draw those letters 3-dimensionally. I could explain it to you here.... But its easier if you research it on your own or have someone show you in person.