How to create your own formulas?

I have taken math to differential equations for my studies. So I am not an expert in math by any means but have taken more math than most. In class they just feed you equations and ask you to solve them. But what if I want to apply the math to a real world situation? How does one learn to create an equation to help find a solution to a random problem?

This problem could be work related, every day life, something out of bored, etc.

Author: mrk1224