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To get you started, click on the grey "Purpose Object" with the left mouse button. You may then change its name and add a number of "Agents". Agents are (benign or malicious) public display users. Close the dialog by either clicking, dragging, or zooming the grey area next to it.
Now, left-click on the Agent and choose a predefined name from the dropdown box. Each name represents a certain type of agent. Hover an item to see an explanation. Then, click on "Add Threat" to add one or more threats. Threats are things that can users do with or to a public display.
Then, click on the Threat box and choose a predefined name that represents a specific threat type. When you're done, click on "Add Weakness". Weaknesses are specific components of a public display that may be exposed to certain threats.
Open the new Weakness and select a predefined name. In the next step, click on "Add Effect" to add a number of effects. Effects can be regarded as "consequences" of applied attacks (threats) to a public display. You may also add a number of counter measures by clicking "Add Counter Measure". Counter measures may help to avoid (negative) effects of successfully applied attacks (threats).
Since every Counter Measure can be attacked by another threat in turn, you can add another "Purpose Object" to every counter measure. As you can see, building your individual threat model is a cyclic process. Continue until you think that every possible attack (threat) is covered.
You may "Save", "Load", and "Export" your work as a SVG file. Use the magnifying glass on the right or your mouse wheel to zoom in and out. You can also use your mouse to drag the threat model.