The idea is that you write down the circuit with the terminals open, and then find the voltage across them in this configuration, using whatever circuit-solving methods you have at your disposal (i.e., Kirchoff's Laws). To ``short the terminals'', you draw the circuit with a zero-resistance wire across the terminals, and then find the current in this configuration, again using Kirchoff's Laws, etc. Please note that ``opening'' and ``shorting'' supplies and terminals is an imaginary, abstract action-- you write it down to find what the current or voltage would be if you did this.