Let's consider strictly technical aspects of icon making
1. Lines, angles
One of the most important parts of the image is the outline of the object, i.e. rough border, which limits the object from the rest of the image. When working with normal-sized graphics, the artist rarely cares about defining objects with complementary lines. This is not needed because of the size: even non-contrasted objects do not mix into a single whole. Pixel graphics is other story. Two variants are possible: either object and background colors have to be from opposite sides of the color chart, or the foreground should be separated from the background by contrast outlines and shadows. I would like to dwell on lines deeper.
If we return again to the big-size graphics, we see that in order to define edges, we can use all (even the most complicated) angles, Bezier splines and edges. In any case, the line will look perfect due to anti-aliasing. When shrinking the image dimensions to the size of an icon, situation changes greatly. When every particular pixel is vitally important and can change the whole look of the composition, anti-aliasing is simply not applicable. It means that you should think about the possible line slopes.
The angle you select for the line, specifies the step of this line. Because each line consists of basic elements, the combination of which determines its neatness and esthetics.
For instance, a 18-degree angle consists of many small 3-pixel lines combined with a 1-pixel downshift:
This is how almost every line looks like. Basic element with joining and another basic element. Unfortunately, not each slope makes the line look attractive and not messy.
For example, here are 25 and 20-degree angles:
It can be easily known which line is more appealing. The 25-degree angle makes a line containing equal 2-pixel basic elements. The 20-degree slope has a primary element consisting of three parts: 3-pixel, 3-pixel and 2-pixel. The slope is more complicated, therefore, if not combined by additional visual effects, it brings a feeling of untidiness: the basic elements have contrast which is too obvious to escape the human sight.
Now we can come to the first conclusion. The most "appropriate" angles must make as plain primary elements as possible. therefore, the ideal slopes are 0 and 90 degrees. In those, 1-pixel primary elements are joined without any shift and produce smooth lines. Less ideal are slopes which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines can not be even, but the human sight will process the image and provide to the user what you meant to show to him. Also, the mentioned angles can be considered correct (but at a stretch): angles with primary elements consisting of unequal lines shifted by more than 1 pixel.
Now we can discuss another problem. In the last paragraph, I intentionally defined angles with basic elements of 1, 2 and 3 pixels as acceptable. Such small lines, especially when a large number of them is combined, look like solid. But what do we see if we increase the primary elements to 10 or 20 pixels?
Here is an example:
It can be clearly seen that the line doesn't have integrity. It is not a continuous line anymore; it is a set of different lines, situated near each other. Rarely any artist wants its composition to look that way. Thus, we have another rule: if you use minimal slopes, which make long primary elements, it has to be reasoned and employed with great attention.
And, finally, the third aspect I wanted to tell about lines. I did write that the basic element has to be as plain as possible.
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 is made of three lines: 3-pixel, 2-pixel and 1-pixel. The same angle but the look is different. The complicated primary element made the line messy.
Such examples can be made for about any angle, so in your works, try to simplify everything.
It can be stated without an overstatement that the color is the main element of your work; not suiting or poorly balanced colors can kill even the most creative idea. What can we say about the color in icon graphics?
First, there is a certain rule: if the picture looks poor when reduced to grayscale, it means the colors that were selected are wrong. This rule is valid not only for icons but for the whole graphic design.
Second, you will be unable to use common RGB. Of course, you can select GIF colors only, but this limits you greatly. Gradients, shadows, blending and many other tricks will not be accessible to you.
Third, try to enlarge the areas of "plain" color. The more evenly colored parts without blurring and jumps your picture has, the more clear it looks. Pictograms are too tiny and the excess usage of complex effects makes the picture look dirty.
Fourth, gradients. This is a great tool which can improve the look of any project. Besides esthetics, gradients are an ideal way to abolish the "broken line" effect (lines with too long primary elements). But be attentive when applying them since too many gradients can ruin the plain color, and be unable to fit into the gif palette.
Fifth, shadows and flares. All rules for applying shades and highlights in icon creation are completely identical to the rules of the general graphic design. The only ine I have to note: draw everything by hand. Don't apply filters, make all shades and highlights in a separate layer and then edit the opacity. If using effects, you almost never can predict what the result will be. It is unfavorable when you don't control the making of a project. It is awful if you can't control it if creating icons.
And, finally, sixth. Nuancing is the main detail of icon creation, which greatly results the visual appearance of the image. When the green human eye in the picture is sized 1 pixel, increase the color saturation. For instance, instead of 0, 131, 159 choose 71, 195, 242. It is indifferent that the human eye can't have this color; in the first case it will appear like a gray dot, but in the second case it will really be a blue eye.
Thoroughly control the picture, do not let the optical tricks to ruin your design. It is OK if something is not consistent technically, but the entire picture must be flawless.
As an example, I pictured the packet of juice standing in front of me. The picture has plain color, gradients with broken lines, shades and highlights and nuances.
If I talked about something bigger than icon design, this chapter wouldn't fit into a whole book, but the resolution of pictures ;limits fonts too.
The resolution of letters is the main issue as opposed to their fineness. Almost in any case, unless the letters are the main part of the composition, the font size has to be reduced to the greatest extent possible.
In general, almost any graphic studio has its exclusive font with little characters. This font can be created in a couple of hours. You can search the internet and make your own collection of very nice fonts. Primarily, I advice you to download the DS Pixel and Seventen 7Vedi webpage as well as the whole set of fonts from the Lakmus Lab website.
If you don't need a new font, tiny sizes of Arial and Verdana may do. As a last resort, you can create the needed characters yourself.
There are not many rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters should not 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 looks rather readable.