Starting and planning a new project

So, I want to start planning an art project for this summer, ether a comic or a series of art pieces (like a series of portraits) but I have trouble organizing my ideas.

So my question is, beyond those "character charts" or thumbnails how would I go about organizing my ideas? Especially so for plot ideas? (I do incorporate those things, but i want something more)

...write it all down? At least that's what I do.