Often scholars rely on data collected by other researchers and end up, de facto, with the research design developed by the original scholars. But if you are collecting your own data this stage becomes the key to the success of your project and the decisions you make at this stage will determine both what you will be able to conclude and what you will not be able to conclude. It is at this stage that all the elements of science come together.
We can think of research as starting with a problem or a research question and moving to an attempt to provide an answer to that problem by developing a theory. If we want to know how good (empirically accurate) that theory is we will want to put it to one or more tests. Framing a research question and developing a theory could all be done from the comforts of your backyard hammock. Or, they could be done by a journalist (or, for that matter, by the village idiot) rather than a scientist. To move beyond that stage requires more. To test the theory, we deduce one or more hypotheses from the theory, i.e., statements that should be true if the theory accurately depicts the world. We test those hypotheses by systematically observing the world—the empirical end of the scientific method. It requires you to get out of that hammock and go observe the world. The observations you make allow you to accept or reject your hypotheses, providing insights into the accuracy and value of your theory. Those observations are conducted according to a plan or a research design.