At one hour per page, some treat the 24-hour comic as a minimalist excercise how little can you put on a page and still have it be comics but I like to think of it in the opposite way; how much can you draw in an hour?! If you think about it, the answer is a lot! Figuring six panels per page that’s ten minutes per panel. Try it yourself. [Yeah, right now!] Draw a box about 3 inches wide, 2 inches tall, set a timer for ten minutes and see how much you can draw. You might surprise yourself.
As far as planning goes, you can think about it beforehand, but I recommend improvisation as the most satisfying route. Perhaps have some randomizer at startup (like a Pictionary or Tarot Card Deck or a child’s picture book of household objects) to actually prevent you from knowing what the story will be about beforehand. The less you plan, the less likely you are to get frustrated.
Some have found the exercise is especially fun to do in big groups. Some even chronicle the food they ate, the music they listened to, etc. Doing it alone can be kind of bleak, but also have a peculiar allure and can feel like a rite-of-passage, crossing-the-desert kind of thing. No, really. I’m serious! Oh, never mind….
My strongest suggestion is: Do it! It’s fun, it’s exciting, it’s mind-altering, it’ll teach you all kinds of cool stuff about yourself and best of all it’s only one day, so what have you got to lose?
THE ONLINE VARIATION: The above applies to printed comics or online comics with „pages” but if you’d like to try a 24-hour Online Comic that doesn’t break down into pages (like the expanded canvas approach I use in most of my own webcomics) then try this: At least 100 panels AND it has to be done, formatted and ONLINE within the 24-hour period!