Well, that's always the hard bit. You get a lot of lines but no explanations. Very dangerous to play moves without knowing why, but so it goes.
The usual way is to play a game, and afterwards look up where the game deviated from the book. Since you've been thinking about the position already, you probably have some idea why the book line makes sense, and it avoids memorizing lines that you'll never actually play anyway.
NCO is not a textbook : it's a reference tool. You play a game and can compare your line with what's in the book. If they differ, try to understand why, or ask here 
Good. I was hoping I wasn't expect to read it cover to cover
Has Nunn's been updated? My copy is dated 1999 and 10 years is an eternity in opening theory.
NCO is not a textbook : it's a reference tool. You play a game and can compare your line with what's in the book. If they differ, try to understand why, or ask here
Exactly! NCO is only a tool, just like chessbase or Fritz. Owning them doesnt make you a better player but like any handyman, if you can use a tool you are capable of getting the job done.
Best of luck!
Start from page 1 and memorize the whole book by heart. Then hope your opponent doesn't play something that's not in there.
Check out your favorite openings and look through the lines. Every time you play check your game versus what NCO recommended. As mentioned it is an aged opening tome so some "hot" theory may not be included and some lines may no longer have the same evaluation.
if you play correspondence chess, keep the book by you when you make you opening moves. Do not rely on it blindly but check the lines against your common sense.
How does one go about digesting all of this? Are there any particulars lines I should begin with? Where do you recommend I start and what is the best way to extract the most from this tome?