Let's discuss purely technical aspects of icon making
1. Lines, angles
One of the most important parts of the composition is the outline of the object, i.e. solid line, which limits the object from the background. When working with large-scale graphics, the artist rarely thinks about highlighting of objects with additional lines. This is unnecessary because of the scale: even non-contrasted objects do not blur into a single whole. Pixel graphics is other story. Two solutions are available: either foreground and background colors have to be from different sides of the color wheel, or the object should be separated from the background by contrast lines and shadows. I would like to dwell on lines deeper.
If we think again to the large scale images, we see that in order to define edges, we can use any (including the most complicated) angles, Bezier splines and edges. Anyway, the line will appear perfect thanks to anti-aliasing. When shrinking the image scale to the small icon size, situation changes greatly. When each single pixel is vitally important and can change the whole appearance of the icon, anti-aliasing is just not applicable. It means that you have to think about the possible line slopes.
The angle you choose for the line, determines the step for this line. This means that each line consists of primary elements, the union of which determines its neatness and esthetics.
For example, a 18-degree angle consists of many tiny 3-pixel lines combined with a 1-pixel downshift:
This is how almost every line looks like. Primary element followed by joining followed by another primary element. Unfortunately, not each slope creates lines that look look attractive and not annoying.
For example, here are 25 and 20-degree slopes:
It can be easily seen which line is more appealing. The 25-degree angle produses a line consisting of equal 2-pixel basic elements. The 20-degree angle has a basic element containing of three parts: 3-pixel, 3-pixel and 2-pixel. The slope is more complex, therefore, when not combined by other visual effects, it creates a feeling of untidiness: the primary elements have contrast which is too obvious to escape the human sight.
Now we can see the first rule. The most "appropriate" angles must make as plain basic elements as they can. Thus, the ideal angles are 0 and 90 degrees. In those, 1-pixel basic elements are joined without any shift and produce even lines. Less perfect are slopes which form primary elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines will not be smooth, but the human brain will process the image and provide to the viewer what you meant to show to him. Also, the mentioned slopes can be considered acceptable (but at a stretch): slopes 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 tiny lines, especially when a great amount of them is joined, appear as a single whole. But what happens if we increase the basic elements to 10 or 20 pixels?
Here is an example:
It can be obviously noticed that the line loses its integrity. It is not a continuous line anymore; it is a combination of different lines, located near one another. Hardly any artist wants its composition to look inconsistent. So, we have another rule: if you use small slopes, which make long primary elements, it has to be justified and employed with maximum attention.
And, finally, the last thing I wanted to mention about lines. I already wrote that the basic element has to be as plain as it can.
For instance, a 25-degree slope can be produced in the two (sure more of them are possible) following ways:
In the first case the primary element is a 2-pixel line. In the second case, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same slope but the image is different. The complicated primary element produced the line messy.
This examples can be produced for about any angle, so in your works, make sure to simplify as much as possible.
It can be stated without an overstatement that the color is the leading element of your work; improper or badly matched colors can ruin even the best idea. What can we write about the color in icon graphics?
First, there is a clear rule: if the picture looks bad when converted to monochrome, it means the colors that were selected are incorrect. This rule is valid not only for icons but for the whole graphic design.
Second, you will probably unable to use common RGB. Sure, you may use internet 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 "flat" color. The more consistently colored parts without blurring and jumps your composition has, the more clear it looks. Pictograms are too small and the excess usage of special effects makes the picture look dirty.
Fourth, gradients. This is a great instrument which can change the appearance of any picture. Besides visuals, gradients are an ideal solution to abolish the "broken line" look (lines with too long primary elements). But be careful when using them because too much gradients can ruin the flat color, and be unable to fit into the gif palette.
Fifth, shadows and flares. All aspects of applying shadows and flares in icon making are completely identical to the rules of the general graphic design. The only thing I have to note: draw everything by hand. Don't apply filters, make all shades and flares in a different layer and after that edit its transparency. If using 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 can't control it when creating icons.
And, finally, sixth. Nuancing is the key detail of pictogram creation, which greatly affects the visual appearance of the composition. When the green human eye in the icon is sized 1 pixel, increase the color intensiveness. For instance, instead of 0, 131, 159 choose 71, 195, 242. It doesn't matter that the human eye never has this color; in the first example it will appear like a gray dot, but in the second example it will really be a blue eye.
Thoroughly control the picture, do not let the visual tricks to ruin your design. It is acceptable if objects is not perfect from the technical point of view, but the whole composition must be perfect.
As an example, I drew the pack of juice located in front of me. The image has plain color, gradients with broken lines, shadows and flares and tints.
If I wrote about something bigger than miniature design, this chapter wouldn't fit into an entire book, but the size of images restricts fonts too.
The size of letters is the leading issue as opposed to their look. Almost in all cases, unless the letters are the main part of the project, the text size has to be reduced to the greatest extent possible.
In general, almost any graphic company has its unique font with little letters. This font can be crafted in a few hours. You can search the internet and make your own library of very nice fonts. First of all, I would recommend you to download the DS Pixel and Seventen 7Vedi website and the entire set of fonts from the Lakmus Lab website.
If you don't need a new font, small typesizes of Arial and Verdana will work. As a last option, you can draw the necessary letters by hand.
There are not many 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 unrecognizable set of pixels.
However, there are no rules without exceptions and the 5-point Arial looks rather readable.