It’s all up to you, really! Personally I thumbnail a chapter at a time, but I have a rough idea of the contents of all the chapters mapped out in a sort of crazy mind-maze thing I sketched out… it’s a weird way of doing things but it’s working out for me. Some people like to write the script of the comic first, but I find that thumbnailing and scripting at the same time works better for comics since image and writing have to interact. No way is the ‘right’ way though, and how I work isn’t necessarily the best method for you, or for me! I’m learning all the time, and if I find something works better I’ll switch it up. Try out a few things and see what works for you!