Let's consider purely technical aspects of icon creation
1. Lines, slopes
One of the most important parts of the composition is the framework of the object, i.e. solid line, which limits the object from the rest of the image. When working with normal-sized graphics, the artist occasionally cares about highlighting of objects with complementary lines. This is not needed because of the size: even low contrast objects do not blur into a single whole. Pixel graphics is other story. Two variants are available: either foreground and background colors must be from different sides of the color chart, or the object must be divided from the background by contrast lines and shadows. I will dwell on lines in more detail.
If we think again to the large scale graphics, we can note that in order to define edges, we can use any (even the most complex) angles, Bezier splines and borders. Anyway, the line will appear perfect due to anti-aliasing. When shrinking the graphic dimensions to the size of an icon, situation changes dramatically. When each particular pixel is equally important and can change the whole look of the icon, anti-aliasing is just not applicable. It means that you have to consider the possible line angles.
The slope you choose for the line, specifies the step of this line. Because each line is made of basic elements, the combination of which defines its accuracy and esthetics.
For example, a 18-degree angle contains many small 3-pixel lines combined with a 1-pixel downshift:
This is how almost each line looks like. Basic element with joining and second basic element. However, not every angle makes the line look neet and not annoying.
For example, look at 25 and 20-degree angles:
It can be easily seen which line is more attractive. The 25-degree slope makes a line containing equal 2-pixel primary elements. The 20-degree angle has a basic element consisting of three lines: 3-pixel, 3-pixel and 2-pixel. The angle is more complex, therefore, if not supported by other visual effects, it brings a feeling of messiness: the basic elements have contrast which is too strong to escape the human sight.
Now we can come to the first rule. The most "appropriate" slopes must make as simple primary elements as they can. Thus, the ideal angles are 0 and 90 degrees. In them, 1-pixel basic elements are combined without any shift and form even lines. Less ideal are slopes which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Surely, the lines will not be smooth, but the human brain will process the image and present to the user what you meant to show to him. Also, the following slopes can be considered correct (but at a stretch): slopes with basic elements made of different lines shifted by more than 1 pixel.
It is the time to move to another issue. In the last paragraph, I purposely defined slopes with basic elements of 1, 2 and 3 pixels as correct. Such tiny lines, especially when a great amount of them is joined, appear as solid. But what happens if we change the basic elements to 10 or 20 pixels?
Here is an example:
It can be clearly noticed that the line doesn't have integrity. It is not a continuous line anymore; it is a combination of different lines, situated near each other. Rarely any designer wants its composition to look inconsistent. So, we have the second conclusion: if you use small slopes, which produse long basic elements, it must be reasoned and used with maximum attention.
And, finally, the third thing I have to mention about lines. I already wrote that the basic element should be as simple as it can.
For instance, a 25-degree slope can be produced in the two (of course more of them exists) following ways:
In the first case the primary element is a 2-pixel line. In the second case, it consists of three lines: 3-pixel, 2-pixel and 1-pixel. The same slope but the look is different. The complicated basic element produced the line messy.
This examples can be made for almost any angle, so in your projects, try to simplify as much as possible.
It can be said without an overstatement that the color is the leading element of your work; not suiting or poorly balanced colors can ruin even the most creative idea. What can we say about the color in icon graphics?
First, there is a clear rule: if the image looks poor when reduced to monochrome, it means the colors that were chosen are wrong. This rule is true not only for icons but for the whole graphic design.
Second, you will probably unable to apply safe RGB. Sure, you can select internet colors only, but this limits you greatly. Gradients, shadows, blending and many other tricks will not be accessible to you.
Third, try to increase the areas of "flat" color. The more evenly colored areas without blurring and jumps your project has, the more neat it looks. Icons are too small and the over usage of complex filters makes the picture look dirty.
Fourth, gradients. This is a wonderful instrument which can change the look of any picture. Besides visuals, gradients are an ideal solution to get rid of the "broken line" look (lines with too long primary elements). But be careful when using them since too much gradients can kill the plain color, and be not suitable for the gif palette.
Fifth, shadows and highlights. All aspects of applying shades and highlights in icon making are completely the same to the rules of the overall graphic design. The only thing I have to mention: create everything yourself. Don't apply filters, make all shades and flares in a different layer and after that edit its transparency. When applying filters, you almost never know what will it look like. It is bad when you don't control the process of creating of a project. It is worse if you don't control it when drawing icons.
And, finally, sixth. Tinting is the key aspect of icon design, which greatly affects the esthetic look of the composition. When the blue human eye in the picture has the size of 1 pixel, increase the color saturation. For example, instead of 0, 131, 159 make 71, 195, 242. It is indifferent that the human eye never has such color; in the first case it will look like a gray dot, but in the second one it will definitely be a blue eye.
Thoroughly control the picture, do not let the visual tricks to ruin your design. It is acceptable if something is not consistent from the technical point of view, but the whole picture must be perfect.
As an example, I drew the pack of juice standing in front of me. The image has flat color, gradients with broken lines, shadows and highlights and nuances.
If I wrote about something other than icon creation, this chapter wouldn't fit into an entire book, but the size of compositions ;limits fonts too.
The size of letters becomes the main aspect as opposed to their beauty. Nearly in all cases, unless the letters are the main part of the composition, the font size must be decreased to the greatest extent possible.
In general, almost any design studio has its exclusive font with little characters. Such a font can be crafted in a few hours. You can search the web and collect your own collection of very interesting fonts. First of all, I advice you to download the DS Pixel and Seventen 7Vedi website as well as the whole set of fonts from the Lakmus Lab website.
If you don't need a new font, tiny typesizes of Arial and Verdana will do. As a last resort, you can draw the needed letters yourself.
There are quite a few rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters cannot use anti-aliasing. It transforms the text into an incomprehensible mixture of pixels.
However, there are no rules without exceptions and the 5-point Arial appears perfectly readable.