This is all you need to know to do a basic cel, and by 'basic' I mean fan cel. With the advent of Digitally Colored (DC) animation there's lots and lots of cel sketches out there with no cels for poor cel collectors. So this tutorial assumes you have a nice clear screencapture or sketch to work from and aren't doing anything original.
Lines on the front vs. Lines on the back
Do you want your lines on the front or on the back? There are pros and cons to both, but you have to decide because it will effect your entire setup.
Lines on the front:
Pro You can do pretty much what you want on the back and don't have to worry about lines smearing.
Con The lines are more easily damaged without a protective layer of paint over them. Need to choose an ink or pen that bonds well and doesn't flake off or fade.
You will have to print two copies of your 'source', one reversed and one not. Draw your lines using the 'original' setup and then attach your cel lines-down to the second for painting.
Lines on the back:
Pro Paint protects the lines from casual damage. Just in the course of painting a cel, I often have to redo a couple of sections of lines where the ink had been scratched or worn off.
Con Paint may smear when wet paint is applied over it. So you need to choose an ink that stands up well to wet paint and be careful when painting over lined areas.
You will need one copy of your 'source' reversed/mirror imaged.
And don't forget the aesthetics. Personally, I love the look of a smooth unmarked front of a cel. It's one of the things that draws me to cels in general.
Cel + Sketch
Another one of the reasons that I recommend office transparencies to start with is it makes this step really easy. Print out your screencapture according to how you will be doing your lines (above) and put some masking tape along the side that will have the least paint. 2 or 3 little pieces should do fine. You want to be able to 'open' the cel to check to see how the front side will come out fairly frequently. 8.5x11" on 8.5x11" is also easiest because you don't have to worry about realligning things if the cel comes loose. You can still use a larger size, but you'll have to print it out in pieces and reassemble it first.
Be careful handling the cel because fingerprints will interfere with the pen, especially if you're using an ink pen. The line will go from smooth and straight to beaded and ragged. Handle just the edges when possible, but don't worry too much, any fingerprints can be polished off with a Q tip before you start painting.
(image to come!)
Cel + Sketch + Ink
Got everything aligned to your satisfaction? Take out the inking pen of your choice and start to carefully trace the lines. To avoid smudges, travel in one direction. For example: I'm right handed, so I will start in the upperleft hand corner and work my way to the bottom right hand corner.
(image to come)
You'll probably screw up somewhere, but that's fine. Remember; ink cleaner on a Q tip will take care of pen inks, rubbing alcohol will take care of alcohol pens like lumocolor, and mineral oil will take care of paint pens.
Everything satisfactory? All right, move on.
Cel + Sketch + Ink + Paint
If you're using an ink pen, I like to wait 24 hours before starting the paint. It gives it a little extra time to set and makes line bleeding slightly less of a problem. You don't need to worry about lines dissolving under paint if you're doing your lines on the front. But if you did your lines on the front take the cel off and put it face down on the reversed image so that the non-lined side is on top so that you can paint on it.
In the one animation book that mentioned anything about cel painting, it suggested doing colors from darkest to lightest. This is good if your paint is thin and you don't want to have to worry about another color showing through. But it's not the easiest way.
Start with whatever you want. You're going to have to wait until the paint dries to move on until you're more experienced so it doesn't really matter. Once you do pick, how many levels of color detail are there on your cel for the clothes, hair, skin, etc? If there's 3, usually the brightest color is the 'highlight' and is least governed by inked lines and has the most freehand detail. Do this first with your smallest brush. After that, do the 'shadow' or darkest color (if the cel only has two levels of color in this section, do the shadow first). After that dries, do the base color, which is usually the biggest area. Since you've already got all the detail done with the small sections of highlight and shadow, all you need to worry about is staying inside the lines.
(images to come)
Repeat until all colors are complete.
Paint Mixing Tips
Unless you're using cel paints, the paints will dry a slightly different color. Because of this, use a scrap sheet of paint to test your colors on. Do thin small areas and set them under a desk lamp for speedier drying time. This way you can check your colors before you put them down for good on your cel. Removing colors after they dry without ruining the cel is almost impossible, so precaution is good.
A good way to make sure that the colors will not clash is to mix them all from the same colorset. I will get my 'base color' perfected and add a little bit of a darker color to get to the shadow and a little white to get the highlight.
Always mix more paint than you need because you may need it. Matching a color exactly is very difficult, ESPECIALLY after the other color is dry, so don't do it unless you have to! You can buy paint savers in half ounce and ounce sizes that are tiny little tupperware that will keep paint wet. Use these and you can paint at leisure and not worry about colors drying out on you overnight. Some pallettes come with plastic tops for the same reason but they're not as airtight as the little tupperware paint savers.