Source: Fuse School
In this video, the man explains how to calculate empirical formulae in the exact same way that I do it. He explains it very well, very clearly.
I strongly suggest you grab some note paper and your calculator and write it down as he goes through it. Plug the numbers into your calculator to prove to yourself that this works.
The only way to learn this is to PRACTICE it! over and over and over again...
Source: BBC Bitesize
Source: Crash Course
Source: Khan Academy