I do all the panels at once, section by section. I start with a rough thumbnail done many days ahead of time, then any perspective guides for the background; then I clean up the background, do anatomy sketches to make sure everyone is positioned correctly and has the right proportions, then the final sketch with outfits and everything. Then I move onto inking, sink into the depths of boredom for a few hours, flat it all, and do shading with Clip Studio Paint's default transparent watercolor and a custom square brush. Same with any lighting that might be needed, effects differ from type to type, and then I put down the text and move it all to Photoshop to create bubbles and adjustments.
It all comes from a lot of practice and learning what works though. Things like color theory took a lot of bad pages to get it right, as did learning the best brush to ink, making different brushes for objects and organics, etc. It's also good to share colors in the same scenes to have a sense of consistency even if it's a wall that appears repeatedly, I only noticed now I dropped my entire process instead of focusing on colors;;