The first part of a scientific investigation is careful observation of whatever the investigation is about. This means looking at the subject of investigation and seeing what it does, and under what circumstances. Taking notes to be studied later is an essential part of scientific observation. Observation includes identifying the variables of the subject being studied. Variables are any parameter of the subject matter that are subject to change.
Formulating a hypothesis comes after observation. This is an inherently creative step; there is no particular method scientists use to make them. This hypothesis is an educated guess at what the cause of the phenomena being investigated is, or what the nature of the relationship between two or more things is. Generally a hypothesis is a sort of "if... then" statement such as "If X is true then Y must be true." A scientific hypothesis must be falsifiable. That is to say, it must be capable of being proved false.
A scientific hypothesis must make a prediction that can be tested. The third step of a scientific investigation is using the hypothesis that was created to explain the observations to make predictions about other things. If the hypothesis is a good hypothesis, then its predictions will be accurate. If they are not accurate, then it is not a valid hypothesis.
After a hypothesis is formed comes the testing. This takes the form of conducting experiments that will either prove or disprove the predictions made by the hypothesis. The experiments are designed to set up the circumstances of the hypothesis so that the scientists can see if the predictions were accurate. Ideally, these experiments will be repeated by independent researchers to ensure the highest level of accuracy and the least chance of mistake. If the experiments show that the hypothesis was false, then the process is started over. If they are consistent with the predictions, the hypothesis is accepted as provisionally true.