First things first... I almost always start by filling the background in with a dark colour - the watercolour tool works best if you work from dark to light. If you try working on a light background you'll find it very difficult to get a 'smooth' result.

Notice that I use a 'dark' colour rather than pure black... if you add a bit (say 15) of blue, for example, you'll find that when you paint over this with a light colour it picks up some of the blueness, especially round the edges. This is a neat way of adding a hint of extra colour to the end result.

I also quite often scribble randomly over the background with a 1 pixel watercolour brush set to about 175 opacity. As with the added colour to the 'black' fill, the results should be almost imperceptible - until you start painting over it with a large brush - this will pick up the lines and adds a certain amount of texture.

I don't usually have any idea what I'm going to draw before I start... old age has seen my imagination deteriorate somewhat so I need a bit of help to get started. I use a large brush (size=30) at a low opacity (151 - it usually defaults to this when you set the size) and a light colour and just doodle randomly until something starts to take shape. Once something does start to appear in the randomness I add in colour where necessary until I have a rough sketch.

At this point it's possible to add some colour - again I've been very rough and have also chosen fairly contrasting colours just to make the final result a little more striking. I quite often miss this stage out and land up with a fairly monochromatic image... but I like that :)

The next step is to clean up the outline a bit. I switch to the background colour and using a fairly large brush (except for fiddly bits) set to an opacity of about 240 I'll work round the outline and sharpen up the edges.

Once the outline has been cleaned up I start to define details with a light colour (size=5, opacity=175). This is just a matter of looking at the lines that have been created by the randomness of the watercolour tool and highlighting those that I feel will create the details I'm looking for. Think of it as being like looking at the cracks in your ceiling and picking out shapes that look like faces and so on. Obviously the end result will depend to some extent on how your initial doodle turned out... but there are some tricks at that stage to get cetain types of texture - for instance short strokes close together give a good approximation of tubes that can then be picked out at this stage.

Next, I'll usually add some shading using a medium sized brush and lowish opacity (around 140). This helps to further define the parts of the creature and will give a greater feeling of depth. Again I'm fairly rough, though I now have the highlights from the previous step to help guide where shadows should fall and so on.

This is where things really start to take shape. Using a small brush (size=3) with a higher opacity (210) I'll pick out highlights and further define shapes and details. It's still possible to be fairly rough, but not as much as on previous steps - especially when picking out details, for example the tubes around the eye and at the back of the head. Again there is a certain amount of picking out shapes in the 'cracks' here, though a lot of them should already be there - you're just adding more definition...

And now for the most time consuming bit... using a small brush (size=2), opacity of between 220 and 230 and the dark background colour I draw round all the outlines required to sharpen up the details added in during previous steps. This is when you can bring the whole thing into 'focus'. It takes a while and you have to be a lot more careful than with previous stages: whereas a slip-up before could actually add a detail you hadn't thought of, here you have to follow each line perfectly...

Next I switch back to a light colour but keep the same brush size and opacity and pick out highlights: those round the edges of objects and sharp highlights that give some indication of the type of material an element is made of - in this case small sharp highlights on the tubes give a slightly metallic effect. I might also switch back to the dark colour to further define the black lines if necessary.

After adding in a background and a bit of tidying up the image is finished. In this case I just roughed in a very vague background. If I were going for something a bit more defined I would normally merge this onto the front layer, use the White tool to cut out the background and then draw the background onto the bottom layer... and that would probably be the only time I would use the layers.