It’s a hard question. While anyone who’s written for any length of time knows that not all parts of the story are going to make it into the book, it can still be enough to give you a headache. What should I skip? What needs to stay?
Well, the obvious thing to skip is little details that don’t drive the plot. There are plenty of little moments in the story that have to happen realistically but aren’t always mentioned: eating, going to the bathroom, sleeping, traveling, etc. These activities happen over and over again, and they’re not generally interesting (some are even a bit taboo to talk about). In fact, some genres have made such a habit of glossing over things like using the bathroom that it’s become a bit of a joke (Elves don’t do that. Haven’t you read Tolkien?).
That doesn’t mean you should always skip them! There are times when they need to be in the story – either for realism or as a tool for introducing something else (“Don’t Forget the Everyday“). At other times, all those moments do is slow the story down, just like going into detail on some worldbuilding the reader doesn’t need to know (“What Does the Story Need to Be Told?“). As a matter of fact, most of the same rules apply: when the detail helps the plot, setting, or characterization (better yet, all of them), keep it. If not, cut it.
And that’s when transitions come into play (the how part). Transitions can skip time. They can change place. They can do both at the same time! They’re like a magic wand for authors. Think the clichéd narrator from old tv shows: “Meanwhile, on the other side of town, Little Timmy was playing by the well.”
In other words, if you want to jump over one of those events that don’t help the plot, use a transition.
So what if the characters had to walk for 14 days to get to the next exciting moment! Summarize it and move on. Instead of describing every moment of the 14 days (They walked. And walked some more. They said stuff but nothing important.), any self-respecting author’s going to use a transition (assuming the author wants people to keep reading).
“It took 14 days to reach the next town – 14 dreary days of walking and not much else.”
That’s more than enough for that event. Now, if something interesting/important happened in those 14 days, that would be a totally different situation; however, that doesn’t mean you couldn’t skip over them.
Writers can also skip over important plot points or scenes that give vital information. One of the easiest ways to do that is to use point of view. If the main character isn’t present during that scene, it’s not going to get revealed until that character finds out about it. Stories told in flashback can also be tricky with this: all they have to do is tell the story right up to a vital scene and then jump back to the present without finishing.
Or you can simply transition past it. I’m sure you’ve read books where a character walks into a dangerous situation and then segues to the character leaving that situation. Sometimes, the fact that the scene happened is more important than a word-for-word experience of how it happened. Plus, skipping over it can make for a speedy pace and keep the reader guessing.
I guess it all goes back to what the readers need to know to stay in the story and understand enough of what’s going on (but not all). If they don’t need it, cut it.