Along with the diagram Elodin linked, I've also got this one explaining how to approach it. The thing is though, subtractive construction isn't well suited to working with organic subject matter, and really should be treated as an available tool when working on more geometric/hard surface objects. For organics, try to work additively wherever possible, and as Elodin mentioned, our focus isn't so much on reproducing the reference perfectly. It's to construct something that ultimately feels tangible, believable, and solid.