Explain to your audience the two types of research questions---descriptive and explanatory. Descriptive research seeks to answer the question of "what;" explanatory research looks at "why" something is taking place. A research design's fundamental purpose is to ensure the validity of research gathered and accurately answer the original question.
Tell to your audience that research design is not method. Design is independent of method---design incorporates the research methods most suitable to gather the required data and answer the original research question. It is important to express to your audience that while often confused, research design is not a method of data collection.
Explain that research design is a logical structure that ensures that all collected evidence helps eliminate bias when evaluating theories and arriving at conclusion. Design your research to utilize the best methods to collect and analyze data required to answer the original question.