First, you have to define the properties of all the points of the drawing. Remember, in a CAD drawing, each point will have only (x,y) coordinates. However, for our problem, we have to define the type of the point, whether they are fixed points (with a fixed potential) of variable points.
Using this dialog window, you can change the potential of each point. Moreover, you can select whether a point is fixed or variable. If a point is not fixed (i.e. unknown) it is assumed to be variable. We should also remember that if a point is variable, we do not need to specify its potential value, because it will be found after the solution.