Next week at this time I'll be elbow-to-elbow with a group of like-minded creators, all of us attempting to crank out their own twenty-four page comic in 24-hours. See here for the main posting of the event with additional links (or these for 2008, 2009, 2010 and 2012 recaps).
There's some preparatory work that goes into this event, without breaking "the rules" (see McCloud's official ones here) as far as sketching anything out beforehand: assembling the needed materials (like a new pad of paper + extra backup sheets) and making sure the tackle-box of art supplies is stocked (extra nibs + inks, erasers, pencils, white-out, brushes, ruler, tape, assortment of markers etc.).
Mentally it's helpful to have at least a vague concept, a nebulous idea in incubation so as to have on hand any needed reference material. Some of my longer pieces required a lot of such resources packed in from home, like 2012's "Elegy," others hinged upon random encounters of folks on-site to provide models for say 2009's "Peace," and then there were the years I basically illustrated culled and edited text from material such as a cheesecake recipe or, as in 2010's "Tales from an Outlying Area," a very long (and very bad) joke.
As far as the nuts-and-bolts go when plotting the logistics behind such a monumental undertaking, the system that's evolved for me personally over the years of taking the challenge has been allotting set chunks of time to work in waves that cascade across all the pages from beginning to end: as in first scripting everything out, then thumbnailing all the panel breakdowns per page, then penciling all of the pages, then inking everything, and lastly a final cleanup + touch-up pass over everything at the finish.
Some of the other artists I've watched over the years are fans of the tabula rasa style, and/or will methodically complete each panel and each page in sequence from concept to finished ink, one hour on one page. I tend to spend approximately six hours plotting/writing/doodling, six hours penciling, six hours inking and six hours of slop-time to account for talking, taking breaks, eating, stretching/walking around, power-naps and tweaking the art. That translates into spending an average of only a half an hour per page for the drawing stage (combined penciling + inking), which is a little nuts, but part of the challenge is to push yourself. It would also account for the consistently crappy condition (compared to my cartoons) of the completed pages, but given the limitations, it's an aesthetic I can live with. Of course the less time I spend writing + sketching (not to mention farting around doing other things) then it's more bonus time to spend on the artwork. Many artists skip the penciling entirely, going directly to inks, and some abandon any pretext of a linear narrative, or just let the panels and pages unfold organically, opting instead for more of a spontaneous storytelling. Lesson being that I can't ever recall seeing someone draw non-stop for 24-hours: discipline aside if not for nourishment and conversation then there's certain biological functions that will require taking an occasional break.
From raw to refined it pretty much depends on one's individual style and there's almost no end to the various ways to approach such a project as this: be it under the stricture of a 24-Hour Comics event, mainstream industry conventions (a page a day = a 32 page comic per month) or an alternative/independent work that takes years to see through completion. Discovering one's own limitations and pushing the edge just to experiment and see what happens is one of the points behind these sorts of things, along with the companionship, and having at least a little bit of fun.
Realistically, just getting started - or for many just simply showing up - is sometimes half the battle: for many finding any time to sit a while and get a bit of drawing done is a challenge. And not finishing twenty-four pages within 24 hours by no means equates to any "failure" whatsoever... one of the important meta-lessons behind endeavors such as these is that there will (hopefully) be many other hours left ahead of us.