I don't know how others do it, but an illustration of this size (about 10 2-page spreads, plus a cover and various "spot" illustrations) takes me many months to complete (especially in conjunction with a day job). I do it in several stages.
STAGE 1 - Character sketches. This took me several weeks (mostly on the weekends) as there are 5 characters, 4 of whom are based on the author's children. So, altho I'm not a Portrait Artist, and these are not exact portraits, it was still important to the client that there was some resemblance. This took a bit of emailing pics back and forth and adjusting this or that ( the gap btween 2 front teeth needed to be larger, rounder cheeks, not so round on these two, etc.) but eventually they were all worked out. Phew!
STAGE 2 - Really sketchy sketches working on the layout on any paper to hand (envelopes, lined notepaper, the back of a bill...). This was done all along, whenever an idea and spare moment coincided. Sorry - no one gets to see these! They look like they were done by a 4 year old - all circles and odd shapes and scratchy lines meant to represent the characters and landscape w/out actually drawing them.
STAGE 3 - The initial drawing on the actual paper. This is what I worked on this past weekend. I chose a non-scan pencil for this as it's a little waxy - not much dust to make smudgies - and the lines are not supposed to show up on a scan. Still, it has to be done lightly, and not overworked as some will show through.
STAGE 4 - I go over all of the lines I decide to keep with Prismacolor pencils. I match the color of the lines to the actual color of the item (peach for outlines of body parts, brown for the tree trunk, etc.). I lightly erase the extra lines as much as I can. (Some still show, but will disappear in the final stages.) The examples you see here are completed thro this stage.
STAGE 5 - Layering in color (including adding patterns) and blending. Probably one of my fave stages. Scheduled for next weekend!
STAGE 6 - Coloring in the background if it's not going to be left white. (In this case it's not, and I have a special background effect in mind which I'll keep as a surprise!)
I thought I'd like to show my client the progression, as so far she'd only seen the character sketches and must have been out-of-her-mind w/ curiosity as to the actual illustrations. However, this is a one-time deal. I won't do this for every step or even every pic for a few reasons. One - the actual size doesn't fit in a home scanner - it's too big (16.5" long) so I have re-position & scan it several times to get the whole thing sent. Also at this stage I keep it in the pad for added protection and the spiral binding prevents me from scanning the whole thing. That's why you won't see all of one of the children - he's on the side where the binding is. Every time I scan, I take a chance on crumpling up the paper, which most likely would result in having to start over! Plus, an illustration in it's beginning stages can look scary w/ sketch lines showing and smudgies. If you're not used to looking beyond it to the finished artwork, it can make you think it should be scrapped! But, I thought I'd chance it all w/ this first one,just so she could get a little relief from curiosity!
And now I can share it with you, too!