I've scripted 90% of the first 'book' for Blue Star Rebellion, and I have some broad-strokes ideas of what the plots of Book 2 and Book 3 will deal with. I also know what the final, ending scene will be. But beyond Chapter 3, I have no idea how I'm getting to that ending scene, haha. It's so far off in the future that it doesn't matter yet, each book will probably take me about a year to make.
So I'd say I'm halfway between a pantser and a plotter, writing-wise. I wouldn't start drawing a page unless I had the script complete for about the next ten pages because I'm attentive to pacing and so-on, but nor am I willing to wait until the whole story is 100% complete before I start, or I'd never start.
As for planning a page... I visualise the scene in my head over and over, like an animation, then start mentally picking through that for the most important moments to convey action, emotion, characterisation, humour and so on. I then start visualising those as still images, and mentally assemble them together into what kind of panels would work best for them. (This is all mental, I'm not drawing anything yet!)
When I do start laying out a page, I'll lay down a panel arrangement I think will work then start scribbling VERY roughly what's happening in each panel. (With the page I'm working on now, I just wrote words in them to start with!) Sometimes the layout works the first time; sometimes it doesn't, and I'll keep trying different layouts until I stumble upon the one which will work. I don't start sketching neatly until I know where everything is going and where the dialogue will fit.
It's weird when I think about it, that so much of my comics process is pure visualisation rather than pencil-on-iPad, but it's what works for me! It also means that a substantial chunk of my creative process can be done literally anywhere. It's nice that for me, 'relaxing in the bath' can be an enormously productive thing to do!