I don't see the point of doing both a detail script AND thumbnails, unless you work with someone. But I think it can help a lot in the long time,
Without working the plot at first, it inevitably ends up unfinished as nothing makes sense anymore and the drawing process become difficult...
Still my writing is very rough (usually I can't get the dialogues alright so I know I WILL have to rework like 5 times)
I like to use adhesive notes so that I may work the timeline of the story like some jigsaw puzzle and place each scene and not forgetting one in the proces (and adding one is always easier that way xD
So there are "action" notes, "plot information" notes, and I put them on the different settings of the story.
Then, I draw thumbnails of each panel until pages are composed to my liking.
Then, I cheer up because I was so productive !
Aaaaand when 6 months later I'm actually on that part of the story and reopened my thumbnails, I hate my past self because I can't distinguish whatever it was I draw that f**** day xD