I used to just wing it as well when making comics (or partaking in any sort of creative exercise), but I've been much more productive since learning how to plan ahead and organize.
My process might differ from others since my comic is mainly gag-a-day with the occasional extended story arc, but here's the gist:
• Writing the script. I keep a word document filled with scripts for individual comics. Usually I let them come to me during the course of the day, when I'm daydreaming at work or lying awake in bed or whatever. I'll jot them down in a notebook or on my phone and then add them to the log later. Sometimes I'll be lucky enough to have the script pop out of my brain fully-formed like Athena from Zeus, but more often I'll only have a setup with no punchline, or a punchline with no setup, and I'll just write down what I have so I can hopefully develop it later. By keeping a log like this, I ensure that I always have a pool of scripts to draw from, and that once I've finished one strip I'm ready to start on the next.
• Scheduling strips. My comic updates regularly (every six days), so I plan ahead and keep a schedule of upcoming strips I want to work on. For example, knowing my comic will update on January 2nd, 8th, and 14th, I dig through my script ideas and pull up a few that I think will work well together. Generally I try to find a balance between the four main characters of my comic. E.g., on one day I may have a strip focused on Character A, then the next one will feature Character B and D, and then the next will have Characters A, C, and D, and so on, so that way none of the main cast falls into the background. That's my typical concern, but another webcomic by another author may have different ones. Organize your strips by whatever metric you feel is most important.
• Comic layout. Once I'm ready to work on an individual strip, the first thing I do is determine the layout. This is a very important part of comicmaking that I didn't quite grasp early on. Think about the flow of the strip, the focus of each panel, how dialogue will be divided and arranged, et cetera. I usually try to determine the size and position of each panel, as well as the shape and location of word bubbles, before getting to work on individual panels. None of it is set in stone at this point; I often end up enlarging a panel slightly, say, or adjusting the wording or shape of a dialogue bubble. But having the general layout determined beforehand is immensely helpful.
• Drawing individual panels. The meat and potatoes of comicmaking. Having a basic idea of the size of a panel and what parts will be covered up by dialogue, I can start drawing. Typically I'll start with a rough sketch of the foreground (whatever the focus of the panel is, which is usually the characters speaking), then a rough sketch of the background. (I used to do rough drafts with pencil and paper before switching entirely to digital work... being able to resize and reposition things on the go makes the process sooooo much easier.) After the sketch looks good, I'll ink it, and then color it, and then shade it. When the panel looks good, I'll add it to the main image and move on to the next panel.
• Finalizing the strip. Once all the panels are done, I do the last bits of tweaking: adjusting panels as needed, drawing borders for each panel, drawing and positioning word bubbles, and all that jazz. Once everything is complete, I save the files (one hi-res original, and one downsized version for uploading), and upload the strip for future release!
• The last touches. If possible, I highly recommend maintaining a buffer. I try to keep a buffer of 2-3 months for my strips. It'll give you room to breathe and allow you to continue doing regular updates even if something comes up in your life that prevents you from working on new strips for a while, or if you want to take a short break from drawing. On top of that, having a couple months to look over your strip before it's released to the public may allow you to notice issues that you missed before. Maybe you misspelled a word, or maybe you a character's head is a mite too big, or maybe you forgot to color a spot under someone's arm. (Stuff like this happens to me more often than I'd like to admit.) If you have buffer, you have time to catch these mistakes and fix them early, instead of waiting for a keen-eyed commenter to point them out to you.
Them's the basics... This is what has worked for me, and I hope it's helpful for you too.
I would also second (third? fourth?) Scott McCloud. Making Comics and Understanding Comics are great resources, and they got me to think about a lot of things I never considered in comic design prior, particularly with things like flow and panel layout.
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