Ok, once you have a sketch and are ready to line art it, simply go to the piece of paper with the calligraphy pen on it. It'll make you a new layer with different options for the pen and editing tools. Once you've done that, it should look like this:

http://i46.tinypic.com/653789.jpg

It helps if you make the sketch layer a lower opacity, especially if you're doing your lines in the same colour as the image below.

http://i48.tinypic.com/33lctab.jpg

Ok, the image above is the curve tool set to size three. The curve tool is useful for if you want to keep the line the same thickness all the way through, it's also good if you need to move your hand away during the stroke, because it puts down a point whenever you click, tap or put pressure on, which you can easily undo without loosing the whole line.

http://i49.tinypic.com/2latyc3.jpg

The image above this paragraph is set to pen tool and size three. This reacts to pressure, so if you go light it has a thin line, if you press down hard, it does a thick line. This is also good if you want nice clean sweeps.

If you mix both types together, you can get some really lovely results. The image below uses both of the above examples, if you look closely enough you should be able to tell which is which.

http://i50.tinypic.com/1okv3p.jpgp

This trick can work on all layers, and not just the line art ones.

Click on the piece of paper, this should give you a new layer. Then tick in the clipping group. Make this ABOVE what you want to colour.

http://i46.tinypic.com/34ys7dz.jpg

Then, on your new clipping layer, colour it in!

http://i45.tinypic.com/300sawj.jpg

The above shows you how that works quite plainly, but you can do some real fancy and beaut stuff with it too. You can also use this to colour in really quick with the bucket too.

http://i49.tinypic.com/2pre2aq.jpg

Swag.

If you're going to make an image smaller you should consider how the image is going to look when it's smaller, you lose a lot of detail unfortunately. I learnt this the hard way from making pets at DNAdopts.com . We shade in as if the light source was BEHIND the pet, so basically we colour in the middle. This technique can be used for the lighting as well.

I make a new folder to put all the shading in, because it can get messy if too much is going on.

http://i46.tinypic.com/2zgubfb.jpg

The above image is coloured in with black, but since it's opacity 56 instead of 100 it looks a lot lighter. The opacity is good to fiddle around with because the shadows look more or less pronounced based on the colours underneath. Heck, you can even make a clipping layer on this one to change the colours to work better with the base colours if you want.

http://i47.tinypic.com/1933f7.jpg

You might want to do more then one layer, for added depth. Below is a comparison for how I started doing them (shading according to the big picture) in respect to this shading technique (shading with a larger colour difference for the smaller image). Hopefully you can see which ones look better.

http://i46.tinypic.com/fqnpe.jpg

First of all, you want to keep the different colours, seperate. By this I mean, have fur on one layer, hair on another, bands on one, eyes on the other, where ever the pet has different colours. Like so-

http://i47.tinypic.com/3312355.jpg

Once you've got that, you can save your first colour for that pet. Then, go back to the layer, now, you can always select the area again with your wand tool, or, you can make a new layer, clip it and bucket tool the colour there. The coloured lines tutorial has info on clipping groups if you don't know what I'm talking about.

http://i45.tinypic.com/15nlmrc.jpg

And that's how you make new coloured pets without much effort. You can do a whole set in only a few more minutes then the original!

The first layer of Sai is always transparent, even if it looks like it's white. Ok, once you have your cute little hurriedly drawn butterfly, or whatever image you want to save-

http://i46.tinypic.com/11uxjdt.jpg

It doesn't need to be flat either, you can even put a half opacity layer on, make it kind of look like a glow (with the airbrush tool)

http://i45.tinypic.com/2lldfo2.jpg

All you need to do is go to file, save as and choose Png format.

http://i48.tinypic.com/fp85so.jpg

From there choose 32bpp. Choosing 24 will save the image, but not as a transparent image.

http://i46.tinypic.com/dolau8.jpg

The image should then save as transparent.

http://i50.tinypic.com/241pbts.jpg

You can even have fun with the image below this by dragging it across your screen and seeing through it :3

http://i50.tinypic.com/2diloqv.jpg