The first step is make sure you have line art of your drawing. Clean and neat line art is essential to having a nice drawing. If you're not sure how to outline digitally I have a tutorial (it's for microsoft paint only though) on it; outlining in Sai is similar you just have to open a "linework" layer. Look for a button that has a sheet of paper with a pen over it.
I accidentally already start coloring but here's how to do it. Make a new layer and name it (you'll have a lot of layers so naming them helps to keep it organized).
Go to your line art layer (it's the one I have highlighted) and take the magic wand tool that I have circled in red. Click spots you want to fill with a color, so for example I clicked inside the skirt since that's what I'll be coloring. It should highlight all the selected area in blue.
Do this with each section until your whole drawing has base colors down.
While the area you want colored is still highlighted, switch to the layer you'll be coloring with. For me this layer's name is "clothes". Take the bucket tool and fill in the entire area. If you did it right then only the selected area will be filled.
This is what your drawing should look like once all the base colors have been added in.
It's time to color in white areas if you have any, if you don't then you can skip this step.
Coloring white on white is tough so I make things easier by making a new layer and setting it below ALL my base color layers and fill the entire canvas with a random color, the darker the better. Now you can see what areas you need to fill in with white. Repeat step 2 and 3 to color in the white areas.
All white areas have been colored in now. You'll want your white areas to have their own layer (instead of not coloring in white at all) so that you can shade in those areas later.
Time to clean up.
Even though you've used the selection tool and bucket to fill in areas, there's going to be some tiny spots that haven't been filled in with color. It's important that you manually color in these areas, preferably with the pen tool on a small setting. Leaving white spots can sometimes be noticable and it's no fun to see them after you've finished your drawing.
In this step you also may need to clean colors that got out of the lines. That's fine too, just take your eraser and erase those spots in the respective layers.
Much better! The base color of your drawing should look clean now. :)
Time to start shading. I recommend choosing a different color like I did (I chose salmon even though the base color is a yellow-orange color) or slightly darker shade.
You can start with any layer you want but I'm starting with the hair. There's plenty of ways to do this, this is a bit more work but I'm picky about how things look. Make a new linework layer and set it above the base color (so this one is set above "hair") and then click "clipping group", it's circled in red so you can find it. What this does is that it clips the new layer to the previous one and you can't draw outside that specific color area. So I can't color outside the hair layer.
Take the pen or curve tool (you can use either) and start drawing out where you want your shadows to be placed. You'll be using this as a guideline as well as something to fill in when it comes time to fill in those areas.
**I cannot teach you where to place shadow and shading on a drawing because it's not something I'm too good at and usually guess. It helps to choose a light source and imagine where light would shine down on your character and also where light doesn't touch.
Make a new layer and put it above the base color layer (hair) and below the layer made in the previous step. Make sure you clip this layer too. Select the layer from step 9 and on your keyboard hit the keys "ctrl" and "E". This will merge the layer onto your new layer.
I've already started filling in areas but what you're going to do is outline areas (with the pen tool) that you drew out in step 9 and fill them with the bucket tool. Make sure that when you're outlining that you go around the entire section you want to fill or instead of just filling that area, it'll fill everywhere.
Once you're done filling in all the areas you want shaded it's time to do a touch up.
Remember how I told you when shading to choose a different color or slightly darker color? Here's why. Right now your layer setting is on "normal" but we're going to change it to "multiply", it should turn your shaded area even darker than before. That's what "multiply" does. At 100% it'll be way too dark though so adjust the opacity to your liking. I chose 48% as a decent opacity for the multiply layer.
Continue doing steps 9-11 with each base layer color. I moved on to the skin layer and started shading in.
If you're on the skin layer like I am and want to add some blush to your character's face make a new layer, set to "multiply", and clip it (my layer order is "skin", slight shading, hard shading set to multiply, and then "blush" set to mulitply). Take a pinkish/reddish color, grab the airbrush tool and set the density to 33 like I have and the airbrush the skin where you want it blushed.
I figured I'd add in a step for how to do eyes. Make a new layer over your eye base layer and clip it, name it "highlight". Pick a lighter color than your eye's base color and fill in a small section like I did. There's also another way to do this but that's for another tutorial, it involves changing layer modes.
Now you're going to want to give your eye some depth so make another new layer and set it above "highlight" and clip it. Since we're using the multiply mode I used the same color as my base color for shading; as you can see it's darker. Color in the area you want darker with the pen. Now take the water brush and smooth it out a bit. Can you see the difference?
See the highlight in her hair that's sweeping across? Anytime you want to add highlights to your drawing, you're going to make another layer for that, make sure to clip it like always. Set the layer mode to "screen" and pick white or a light color. Color in the area you want highlighted and then set the layer's opacity down so it's not overpowering. I have it set to 45% here.
Continue on with the previous steps until all your layers are shaded and highlighted (if you wish).
Now you'd done. :D