Issue with adding custom rule

Hi,
I try to add my custom rule according to what you wrote on the readme.txt file, but unfortunately it doesn’t work…
I have some questions :

What do you mean exactly by “First, you have to create a custom rule in the **filename structure** option using curly braces, like **{my_custom_rule}**. You just have to write it, in any position you want.”
– How and where do you “create” a custom rule exactly ?
– The filename structure : do you mean the class-filename-strucure-option file ?
– And if yes, does it mean I will not be able to update the plugin anymore since I’ve modified it ?

Thanks in advance, and by the way great plugin !!