I'm not an expert but I'd like to share what I've learned so far.
Note: I draw everything digitally cuz it's way easier for me
--- First off, the thumbnailing/storyboarding. I use a default canvas then divide it in 4 parts, this helps me get an overall view on how the panels will flow. An example below (sorry in advance for how messy it is xD but hopefully, it can help you get the picture xD)
Some tips: If you're drawing a vertical format comic, don't draw your storyboard panels like a traditional comic book then just moving the panels vertically afterwards. The vertical format is it's own thing as I've observed.
--- Drawing the comic. I mostly draw each PANEL on a separate file, like this.
It's done in 2400x3840 at 350 dpi. This size is kinda optimal for me cuz you can resize this at 800x1280 at 350dpi in proper aspect ratio, so no distorting or manual adjusting needed. Sometimes though when you need a really long panel I do it like this one.
I draw this on a 2400x7866 canvas (Take note, it has the same width as the normal length canvas I draw on, it helps in exporting later) then distribute this panel in 3 separate files, see the blue square overlay? That's the cutting point I use so it will appear like this in 3 separate files.
File 1:
File 2:
File 3:
I guess at first it's tedious? But Webtoon (and Tapas) has file limits in them, so a really long canvas would exceed this limit (happened to me) and remember the 800x1280 limit too, a long canvas wouldn't work (atleast for me)
--- Exporting the pages. I don't use Photoshop much, I use Manga Studio Ex5. Trust me it's very helpful in comic making since you can save your comic into 1 file like this:
After finishing a batch of pages, you can go to Story>>Batch Export. Which will land you to this menu
This function allows you to adjust every page you specified uniformly. I'm pretty sure all of us draw in a bigger canvas then resize it to Webtoon or Tapas' page requirements. So if you use this function, you can resize every page you want with one click.
I think that's pretty much it. Sorry if it's a bit messy but I hope this helps ^^