Heraldry, in period, was a method of identification that allowed the illiterate peasants that made up a large proportion of medieval armies to identify who they were fighting for. In the middle ages it developed into a structured art form with a series of rules or criteria that had to be adhered to.
To create an SCA device is relatively easy once you understand the basic rules of period heraldry. Each device is registered to one individual, group or household so no two are alike.
There are five colors: Red (Gules), Green (Vert), Blue (Azure), Purple (Purpure) and Black (Sable). There are also two metals: Gold /yellow (Or) and Silver/white (Argent). There are also several patterns of colors and metals called furs, and treatments. The most basic of heraldry rules, states that you may not place a color on a color or a metal on a metal.
The background color of your shield is called the field and that is a good place to start. It may be either a metal or a color or fur. The field can be broken up in several ways: Per fess (horizontally), Per pale (vertically), Per bend (diagonally) or Per chevron, among others.
A charge is an object, animal or shape. Orientation of charges must also be taken into account. There may be one or multiples of any one charge. The variations are almost limitless so with a little practice you can design a device that is both pleasing to the eye and clearly recognizable.