The second step is to identify any concepts that could be expressed by similar or related terms (i.e. synonyms). As you identify related terms, you can make a list of them under each concept.
Continuing with our example, consider the historical context in which the play Antony and Cleopatra takes place, during the period of the Roman Empire. The term "power" could be expressed using words such as politics, political, rome, or roman, so you can list those words under "power". Your list will now look something like the following:
power, politics, political, politician, rome, roman