How to write a custom rule in SQLFluff?

I was learning about SQLFluff and how to lint using it. While linting, it used only the default rules for checking. But our product need different customised rules for linting. So, I planned to write a custom rule in SQLFluff.

I was trying to find how a custom rule is written. But was not able find it. I need the step wise procedure to write a custom rule in SQLFluff.