You create the "flow" of data. Often you are left with a two state answer such as true/false, on/off, right/wrong, etc. You simply create a branch that handles each state until you reach the end where a conclusion has been reached. In cases where there are more than two states, you begin branches that handles each of the possible scenarios. Each decision is like an "if statement" where "if" at that junction the answer is true, you follow another path on what to do, or if the answer is "blue", you create a path on what to do for an answer of blue.