Tools:
Oekaki (I prefer shi-painter)
Mouse or tablet (I use a mouse, a tablet will make the job faster but not necessarily better.)
Time (It does take a while, sometimes up to 4 hours to finish a piece.)

Difficulty:
Intermediate to Advanced.
This tutorial uses a few advanced skills. But most of the elements are intermediate, it just depends on the individual's artistic abilities.

Start the picture off by using the pen tool to sketch out the desired outline. (The oekaki tools tutorial will tell you which tools I'm referring to.) If you're not using shi-painter but another program such as OekakiPotato, use the watercolour tool and carefully turn the alpha (opacity) to 250 or 245. (Which ever is one mark down from full opacity). If you use this method you'll need to remember to go over your lines a few times later. So I recommend using the shi-painter tool at http://www.kenshin.net.

Add basic areas of colour using the watercolour tool. (Fill works just as well.)
IMPORTANT: Make sure you do this on a layer below the lineart, or you'll cover it. If your lineart is on the bottom layer (Layer 0) then go to layers and click/drag Layer 0 over another layer on top and choose 'Shift' not merge. If you use shi-painter you can do this for every new colour you add. (For older Oekaki tool users, use Copy to move your layer 0 graphic to layer 1 then colour in the areas on layer 0.)

I usually move on to the skin at this point. Add darker colour around the edges of the face, around the cheeks, under the lips, to one side of the nose and near the corners of the eyes. I chose red here because of the general colour scheme of the art, this darker colour can be any colour you wish. Even green or blue, depending on the feel you want the art to have. Just remember to blend. (If you don't know how to change colours check out the Oekaki Tools Tutorial.)

The scar is made more life-like with a hint of red and a small amount of 'spread' on the colour around it. The lips are coloured so that the upper is darker (it is slanting in such a way that light doesn't fall on it.) and the bottom is lighter, with points of light. The eyes are left alone.

In this picture I chose to move on and add a bit more colour to the metal piece on the character's headcloth. I have the tendency to not fill in areas completely and here, the background colour became part of the metal's shadow. It is important when you want something to appear beaten to have uneven lines of shadow and areas where it seems the material is broken up, or in this case, designed with holes for tying cords or other things.

The high portions of the cheeks are given more light with a lighter shade dabbed in. The strokes are not blended since I feel oekaki is not exactly designed for that effect. Small white lines are added to the stitches on the scar to give it a more 3D feel. Slightly darker shadow is also added for the lips and nose.

I've decided to make the head piece a red metal. The strokes were a wider brush, lower alpha and done in a single stroke. The cracks were added afterwards. A crack is easy to do, choose a colour that is almost black then draw the crack and add white for lighting where appropriate. (Any edge facing towards the light source.) White is added to the rim of the holes and detailing and darker shadows are added to make it look more 3D.

The eyes start with a base colour, usually white, some people prefer to use gray or other colours, it really depends on your colour scheme. I chose red for the eyecolour, the eyes are first filled with the colour and the detailing is added afterwards. The iris is a darker shade of red and the line around it is the same. If you look closely there is a pale, almost white line under the eyes, this is to show definition and light. It's not necessary, but I prefer to use it.

More shading is also added to the skin just under the fringe. Again the same red scheme is used. More detailing is also added to the metal head piece.

IMPORTANT: It is about here that some of the colouring is in a layer above the lineart to clean off the sketchy edges. Shi-painter lets you have many many layers, the old oekaki tools do not so you will have to draw on your lineart layer. Do NOT merge lineart layer down with the BG layer, you still need them separate to do colouring underneath.

Carefully add more shadow, doesn't matter if you go over the lines since the hair colour that comes afterwards will cover it. The shadows might seem heavy now but once the rest of the picture is coloured it will be fine. It is very easy to make the shadows too light and have the picture come out looking flat. The corners of the eyes are made darker. This is just a style preference, it's not necessary.

I added darker rims around the eyes because I felt they needed more definition. Eyes are a very important part of the picture, they are often what gives the character its soul.

Basic colours for the hair is added, I didn't want the entire picture to be red so I added portions of white at a low opacity. Darker reds are used to add definition to areas of shade in the hair. Shading over the top of the sketch lines (On a layer above it) now happens, and you can see that the face is now a lot tidier, most the stray lines covered by colour.

Using a thin brush, add strokes of lighter or/and darker colours to the hair, this makes it more 3D and gives the whole thing a less flat look. You can also add loose strands on top of everything. These need to be on a top layer. And in the old tools, on the lineart layer.

Even more strands are added, and some of the sketch lines are cleaned up further. It is important that the strands fall in such a way that adds to the picture and makes sense. Hair can be unkempt, but most of the time they fall a certain way that is similar to the strands around it.

The base colour for the headcloth is added, like with most of the piece, the background colour is used as the shadow. It's important for cloth that the strokes aren't even and blended so that it has some texture. Colour is added to the leather belt holding up the headpiece and again white is used to represent lighting on the stitches and around the holes.

Shadow on the cloth is added first, then the red weave pattern. Make sure you know which way you want the cloth to slant and work out how the folds and direction will affect the patterns on it.

I added a few lines and points of light to the cloth so that it has more definition and dimension. It is important to know how thick you want clothing to appear since how thick/rounded your shadows/lighting is will affect how thick it will look.

More shadow is added under the chin, cleaning up the sketch lines on the neck. And the base colour for his cloak is added.

The cloak is coloured and more lighting is added to it.

I like the idea of red and silver metals for this character. Note the line of light on the upper edge of the metal shape. the thickness of this line can also indicate the thickness of the metal. I also like to use a line of darker shade directly under it to show a 'lipped' edge rather than have the white lighting line go directly over the shading for the rest of the metal. Lighting is very important here as it shows the shape of the piece. Because it has a slight 'bulge' the light falling onto it is represented in a circular formation in small strokes.

Again cracks and holes are used to add detail. I also added a bit of red to the holes for an extra touch.

Backgrounds sometimes happen by accident, the background for this piece was originally going to be an exotic city scape, and that's why there are those lines that look like wonky gridwork going up.

This is when I added lights to my 'city'. At the same time I added lighting to the eyes to make them come alive. There are several ways to add light to the pupils, I just prefer the circles and stroke since it follows the shape of the eyes. There is also gray added to the whites of the eyes to make them look more 3D.

The 'city lights' were created by using circular strokes of yellow then putting a larger opaque white dot in the middle.

Some holes for detail are also added to the headcloth. And to tidy the picture up I carefully add a white outline over the lineart layer.

More lighting is added, and some purple blue light intended to be part of the skyline.

I had the idea suddenly to turn My city into a garden, and everything in the background takes a drastic change. The skyscraper lines become trunks, and the lights are fruits growing at the top. Branches are added but the whole thing is hazy, leaving the character the only defined element.

The finished product. The white outline defines the picture, areas where more definition is needed have neater, slower strokes of black line to add what is needed.

Sometimes signatures can be just as decorative as the rest of the picture, as long as it matches the colour scheme and designs in it. Here I wrote the first letter in my initials and added swirls to it, then lines and shadows of red.

I hope this tutorial was helpful. If you have anymore questions, don't hesitate to email me.