Instructions
Determine the object of the game. For a theme based around the Underground Railroad, it would be fitting to make the object of the game to help the most slaves to safety. Alternatively, the object could be to build the best hidden route or to help escaped slaves hide from authorities.
Design mechanics in the game world that correspond to the theme. If the object is to help slaves escape, you could move figures indicating escaped slaves along a track to freedom. If the game's object is to build a hidden route, you could have players compete for parts of a track that they place on the game board to build a route to safety.
Include elements of competition. You could include a mechanic wherein players direct authorities away from their own hidden slaves and towards the slaves of your competitors. You could even have one of the gamers playing as one of the malicious authorities who are trying to hunt down the escaped slaves, and the players must work together to outwit him.
Create mechanics in the game that utilize American Civil War era history. For example, if the game has a deck of cards to determine random events, you could write facts about the Underground Railroad on each card. You could name important places on the game board after real landmarks in the American Underground Railroad, or you could name tokens after important individuals who participated in the illegal slave liberation movement.
Create the individual components of the board game. Use construction paper or poster board for the playing surface, action figures or cut-outs for players' markers and include an element of randomness, such as dice, cards or a number spinner.