AA is a mathematical function (roughly equivalent to a 'box filter') for simulating sub-pixel detail (having a shape 'partially cover' a pixel). the pixels of the line/shape you are antialiasing can be considered as plotted with a given opacity (say you're plotting black on white -- black pixels would be 100% opacity, white 0%, mid-grey 50%), according to how much of the infinitely-precise shape you are trying to represent falls into that pixel.
Generally the other important framing device is priority: you are always antialiasing one line or shape onto another, so you have to decide which one is in front.

More direct instructions:

Imagine you have gridded paper, and draw a filled silhouette on it. The opacity of each pixel is equal to the proportion of black in the grid cell.
That's the basic idea that you can get a sound theoretical understanding from -- Go play with some gridded paper and translate the shapes into greyscale pixel art.

After that there are the questions of workflow and the more technical details of how eyes work that prescribe the avoidance of certain patterns and clusterings, which patterns can be usefully exaggerated, and why the '50% mix' of two colors is often not a straight averaging of their RGB values. For those aspects, on Pixelation you can find several tutorials, plus Helm's 'ramblethread'. My advice though is to only think about that stuff after you really have a good grasp on the -basic- techniques.


Edited by neota - 26 October 2012 at 1:36am

I'm still not sure I understand...

I have seen that image from ptoing, but I can never make my lines appear smooth, they just look banded or blurry.

Maybe I should give you a better piece, here is one I made with a little AA. Is what I'm trying to do as AA working?

OK, I gave it another shot. I think it looks smoother now.