TRSE: How To Draw Tutorials:
How to Make a Simple Manga
Step 2
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    Now think up of the ideas that will make up a story, and then run the story in your mind like a movie. DON'T think in words! That will limit your ability to draw a manga. Just think in pictures and pretend that you're in a theater with a movie version of your manga showing on the screen. Let's take one of my Senshiology mini-manga titled "What's Up with the Colored Fingernails?" as an example.  I  had imagined how that mini-manga should run, and write down a script like this:

Venus: "Venus Power!"
    Her hand up in the air, with her fingernails glowing in orange
    Venus get in a position
    Venus pauses, staring at her gloved hand.
Venus: "Hey Mercury! Why would our fingernails be colored when they're covered by the gloves?"
Mercury: "Good question. It may have to do something with our powers." (pondering)
Venus: "Let's give it a try." (removing her glove)
    Venus jabs her finger, her fingernails glowing.
Venus: "Venus Crescent Beam!"
Venus: "Did you see that?! My fingernails glowed before I fired the beam!"
??????: "OW!" (Venus and Mercury jump)
    Jupiter comes up to glare at Venus, her butt smoking.
    Venus grin apologetically.
 Mercury: "Oh of course. Our colored fingernails act as a focus for our powers. Just be sure to watch where you aim."

    By looking over the script you'll be able to see how a story works, and such script will allow you to make any revisions will improve the story better. If you want to make a story even better, give a rest, and you can then come back to it some time later. Our ideas and perspectives always change over time, and such changes will result in improving the story. For example, my Tuxedo Mask: Spirit of the Moon was totally different from what I came up with a year earlier when I first completed Sailor Mercury: Beryl's Curse. A year of brainstorming and changing perspectives made Tuxedo Mask: Spirit of the Moon much better than before. My Sailor Moon: Serena and the Red Dress went through three major revisions before I'm satisfied with it due to new and better ideas.