There are many ways, and this is how I would do it:
The first thing is, finish your first draft. There is nothing you can do until then. Any planning or thinking is a waste of time before then.
Then, create scenes from your work. It doesn't matter if you are a plotter or pantser. Take what you have written (and not what you wanted to write), and write down all the scenes that exist. For example, "Scene 20: Hero chases evil wizard to recover stolen magical sword."
You can write these scenes in a normal text editor. Then look at each scene and ask yourself:
Does this scene have a good conflict, that matters? I.e., not a conflict like James can't find the remote control to the TV
Does this scene move the story forward? If you have a cool action scene which does nothing for the story, change it or get rid of it
- Any scene in which the main character is passive, get rid of it. So no scenes where the hero is moaning about his sad life, drinking coffee, or talking about the weather.
The key is, each scene must move the story forward. If the hero has a cool battle with a dragon, but at the end of it he is in the same position as when he started, what's the use? The change each scene brings may be for the better or worse, but it must move the plot.
Another thing I recommend (that not everyone likes), is having a theme for the story, like good triumphs evil. Then you can look at each scene, and see if it clashes with the theme. For example, if your hero beats up innocent people and steals their money, you are breaking your theme. Unless your theme is 'We must do anything to survive.'
Once you have done this, you can decide which scenes to keep, which to throw away or rewrite. At this stage, you can create another scene list, creating the book you wanted to write, and start reworking your draft.
A few iterations of the process (depending on your experience) will get you a book that moves fast, and stays interesting throughout.