It's a process of narrowing down from general to specific, and light to dark. If I would approach this drawing, I would start with a very very light and general "envelope" that maps out the general proportions. You can see I made the shell a bit too tall in my first sketch, so when I moved on to the next step in making things more specific, I adjusted it. I didn't need to erase anything because the envelope is so light it will fade to the back when I place my final lines. Hope that helps :)