Action sequences are hard, and I used to dread them - so I decided to make a sprawling epic of a comic with a bunch of fights in it. Of course. Defeat your weaknesses by running face-first into them!
My process goes something like this.
1. Figure out the sequence of events in text. What happens first? And what happens next? Where is it all supposed to end up? Who wins and who loses? Does the protagonist struggle to win, or is it an easy victory? Who gets injured, and how? Break it down first in the sequence of actions taken/events taking place, then break it down into what's supposed to happen on each page. If it's a fast-paced fight, several actions/beats will happen on the same page. If you want to slow down and emphasise a particular event, you can have a single action/beat on a page.
2. Plan out the environment where the fight takes place. Indoors or outdoors? Is it on flat and level ground, or is it in rough terrain? Are there trees or pillars or something hemming the fighters in? Are there walls? Rivers? Edges over which they can drop? Location matters, as it influences how easy or difficult the fight is, and environment-elements can be used in the fight - if there are rocks around, one of the fighters can use it to throw at their opponent. If there's a cliff, someone can get thrown over it.
3. Come up with specific visual elements you to include. Any cool shots you want to draw? Big bombastic splash pages? Sweet flips? Really iconic punch to the face? Get those ideas down early, and keep them in mind while you plan.
4. How does this flow on the page? This isn't just a reel of images - it needs to work as a cohesive page (for those of us using traditional page-formats) which means you need to consider things like size of individual panels (small panels = impression of happening faster, big panels = slower, etc.), and consider how the action within one panel leads the reader's eye to the next panel, and how it all flows from the start of the page to the end of it. Avoid confusion as much as possible; always be clear about which panels are supposed to be read in which order. Overlap panels where you need to, let characters break through the borders of panels when you want to emphasise something.
I do all of my thumbnailing on paper, and tend to have a smaller sketchbook next to me to scribble alternative takes and ideas in, so I can work out which the best option is.
