Essentially two ways:
1) Derive it – i.e. take a bunch of equations we already know, combine and adapt them to the situation you’re trying to describe and apply mathematical operations until you end up with a solution for your problem.
2) Empirically – conduct a lot of experiments, look at your data, think what kind of equation could describe the data, then fit it.
Generally, if people describe a completely new phenomenon they start with (2) – although eventually someone might come up with a way to derive it from other, more fundamental laws.
E.g. the classical laws governing gravity were found by people who measured movement of planets, then basically tried out equations to explain them, and then tried out different values for the gravitational constant until it fit.
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