STELLA provides four basic graphical "tools" which are linked to create the basic structure of a model. These are referred to as stocks, flows, converters, and connecting arrows.

In the example above, a sample population is represented by a stock. Stock variables are the rectangular boxes which represent a 'reservoir', or state quantity which is conserved. Flows are control variables which are able to alter the state of the stock variables and can be both uni and bi-directional. In this case, there are two flows representing both the births and deaths within a population. Converters modify the behaviour of all three variable classes, and connecting arrows serve as the internal communication 'wiring' for the model. In this example, the properties of a population's birth rate are defined by a converter, which is linked to the model via connection arrows.

To build a STELLA model, the user first defines the model's stock variables and positions them roughly on the computer screen. Depending on the nature of modeled phenomenon and the modeler's preferences, flows, converters, and connection arrows are then positioned on the screen to create the general schematic of the model. Individual variables are then defined by 'double-clicking' on their screen icons which brings up a definition menu in which the modeler enters the appropriate variable descriptors. Text may also be added within a variable definition menu. Variable descriptors include a broad assortment of mathematical, statistical, or logical "built-in" functions, as well as user-defined constants or graphical relationships. A modeling 'time-step' must also be selected which represents the amount of time required to complete one model cycle using Runge-Kutta simulation techniques.