I can only tell you what works for me. I'm used to taking notes while studying so I use that method when writing/plotting/story boarding. Look up the Cornell Method for note taking. It's just basic bullet points, as has been suggested, but I tag points for setting, action, key points, and dialogue. Thusly:
The breakup
-setting-coffee bar inside a book store
-action- Yolanda is holding up a book inches from Howie's face
-key-Yolanda is distraught and angry, which is the only reason she can speak up
-key-Howie's aghast, like he can't believe she's speaking ill of his favorite books
-dialogue- Yolanda "It's because you won't stop reading these!!"
-dialogue- Howie "But.. but you said..."
Kind of like that, but, you know, with tabs. From that you can start building your story boards and make decisions about panel layout, etc. to add or subtract impact.
You can also try Celtx, which is a screenwriting software that's cloud based. Their old standalone is better, and I like it very much. That software and a few sample scripts will get you creating your own from your stories in no time.