So, how do you write an entire novel when you only have small "snippets" of time? If you posed this same question to a dozen different writers (both published and unpublished) and you would hear about a dozen different systems. One system that seems popular is to use sticky notes with scenes written out and stuck on a bulletin board. This never worked for me.
One book that might give you some good ideas is "The Weekend Novelist", by Robert Ray. Whereas his system wouldn't work for me, I still enjoyed the book and especially liked the way he attacked the subject.
Next you should analyze your life. Figure out EXACTLY when and how much time you are able to devote to your writing. My personal experience is that you need at least one undisturbed hour. Some folks start at the beginning and just plow all the way through beginning to end and then go back and edit. No single method is better than another. I can only tell you what works for me and what works for some of my writer friends. You must find (through trial and error) what works for you.
So here's my system--first, I get the plot idea. Second, I write a narrative or synopsis of my story. Third, I create my cast of characters. Fourth, I create a few chapter headings and write a sentence or two as a guide to some of the pivotal scenes. Last, but certainly not least, I begin to write. Of course, nothing is set in stone. An old saying that is sometimes true... "there's many a slip twix the cup and the lip." Yes, sometimes I do wander from my original plot. As the characters develop their personalities, I might change my mind as to the identity of the murderer. The character originally chosen to be the bad guy, might turn out to be good. You just never know what will happen as the word count grows.