How to draw?

How are the standard character (or other) sprites drawn? I know about the shading elements, but the actual scratches... also, how about sense of perspective? I find diagonals so hard to draw.

Also, I've seen some people modelling for character sprites; is this acceptable, or would it look out of place in comparison with the other SRB2 sprites?

To draw the sprites, most people use some form of painting program, or they draw them by hand and then scan them. The most user friendly paint program would, undoubtedly be Microsoft Paint. Though if you find yourself better at hand drawing the sprites than you are at drawing them in paint (like me!) then you should probably draw them. If you don't get them right the first time then just keep trying, because you have all the time in the world to draw those sprites. Lastly you could also use someone else's sprites as a sort of template to draw your own sprites on. Characters made with this method are usually called sprite edits. (Just be sure to give the original spriter some credit for making what became the base of your own sprites!)

Look on YouTube for "SRB2 Coloring Sprites"