This is basically my process when building a comic. I lay out a bunch of empty thumbnails and I work my way from the top. Although it's incredibly inefficient and sorta defeats the point of thumbnailing, I like to make my sketches look detailed so I know exactly what I'm looking at, and doing it like this gives me a great overview of the story, layouts and compositions. Then once I'm happy with how everything looks overall, I take all of those sketches, render each of them on A4 canvases, and I then compile all of those canvases into one PDF. You might notice some changes in the script in the first chapter, and a missing page and panel in the second chapter. This usually happens when I feel that the pacing could be improved, & I rewrote a lot of things while working on these.
From start to completion, these projects take me well over 200 hours. Chapter 2 took me 315 hours in total over the past 2 months!