Now that you have the building blocks in place, it's time to start filling in the gaps. A good story needs more than a beginning and an ending. It’s time to make sure that the rest of your book is linking those two moments together. That connection is what is going to make up most of your plot.
The beginning of your manuscript should be preparing readers for the inciting incident. From the first word of the first page, you should be giving them the information they need to understand what is at stake for the main character during the rest of the book. You need to introduce readers to the main character, any important relationships they might have, and enough information about the world around them to understand the goal that you are eventually going to introduce.
But that doesn't mean showing a stationary picture of the world. Each scene of the book needs to have the characters taking action and attempting to achieve a goal of some sort, even if the goal in chapter one is something small.
Once your readers understand enough about the world, it is time to pull them into the plot with your inciting incident. This should come early on in your book. As soon as readers know enough to understand why the main character has to achieve their new goal, it's time to get the main character moving forward.
Once the inciting incident happens, each scene from there on out should be framed in terms of making progress towards the climax. The main character needs to start taking steps to achieve their goal (active steps, not just making plans to act later) and readers should be reminded of this end goal often.
If you have any scenes in your manuscript that aren’t showing your characters working towards the climax or overcoming obstacles on the way to that goal, it’s time to figure out how those tie into your overarching story. An interesting middle needs to clearly show your readers why it is necessary for the characters to take their next step. Not every scene needs to accomplish something that leads to the climax. That is where your subplots start to come in. But before the characters drift off target from their eventual end goal, readers need to be given a reason why this new, smaller goal takes current priority.
For example, in book 1 when Harry Potter kills the troll, he doesn’t do it in order to stop Voldemort or protect the Philosopher’s Stone. He does it to save Hermione’s life. Those are important stakes. Readers can certainly get on board with putting aside a master plan for a while to deal with a life-threatening emergency.
But that’s not the only reason the scene works. The characters made that situation themselves. Ron's actions accidentally put Hermione in danger, so he and Harry feel responsible to fix it. This provides an immediate sense of responsibility for her safety.
During this scene, Ron has to face the fact that he was being judgmental of Hermione and that he was in the wrong. This realization ties in perfectly with some of the overarching themes of the book. Elsewhere in the story, Harry is convinced that Professor Snape is the Bad Guy because he's "obviously evil." This tie in helps show the children struggling with their flaws and character growth in a wider variety of situations.
The troll fight also brings Hermione into the friend group, which has lasting consequences throughout the entire series. You never want a scene to happen that doesn't effect the story around it. Whatever the characters go into a scene trying to accomplish, the rest of the story should be effected by the events of that scene. Has the main character created this problem in their attempts to solve a different one? Do the events of this scene finally break their ties with their previous life and force them to grow and change into the person they need to be for the final victory? do they gain allies or learn important information?
At the end of the fight with the troll, J. K. Rowling makes sure to take the time to remind readers what the end goal really is. The newly-formed trio starts wondering about who let the troll in and what it has to do with that mysterious off-limits room. This shows readers that the author still remembers where the story is going and promises to get there eventually. Whenever the story takes a significant detour from the main plotline, you need to make sure to show the readers that you still remember the promises you made to them in the beginning of the book and that you still intend to keep them.
Any scene that isn’t working directly towards the goal should be interesting to read for its own sake, but it also needs to be framed in the context of the overarching story. At any point, the readers should be aware of the main character's next step to achieve their final goal, even if they aren't currently working towards that goal. Without these connecting building blocks, you might end up with a series of cool short stories all strung together and not an actual book. This is bad, as eventually readers will start to lose faith that the story is going anywhere and stop reading.
Once you've determined that every scene is framed in regards to that end goal, you are going to reach the climax. After the appropriate amount of set up, the final conflict will occur. The entirety of the climax will likely take several chapters.
Then, at the very end of your book, you need to leave your readers a sense of resolution. Take a moment to tie up any lose ends that weren't resolved by the climax or couldn't be fit into those dramatic, active scenes. Show the readers the new state of the world after the climax and give them a second to breath and accept what has changed before setting them loose. One bad example of this is Infinity Wars. When the movie ended, there was no pause before the end, no showing how the world had changed. It didn't feel real. Which was what Infinity Wars was going for, since the characters all came back to life in Endgame. But if you just cut off like that in your own book, readers won't get to feel like everything is actually settled. They will leave the book itchy and unsatisfied. I know a lot of people who didn't watch Infinity Wars until right before Endgame came out, because they didn't want to be left hanging. So unless you are Marvel and already have your fans eating out of the palm of your hand (and millions of dollars to put into the sequel even if no one buys your first book), itchy readers don't usually make for a good audience.
Applying this to your own story
By keeping in mind your climax during every scene and subplot of your book, you can pretty much solve every major problem your plot might have. Just like there are hundreds of retellings of Cinderella, there are hundreds of ways you can take your readers from the beginning to the end of your story. The important thing to make sure it all stays connected. That strong pull is what keeps your readers turning pages, on the edge of their seat, burning the midnight oil.