How to create custom validation rules?

The only reference I was able to find is Validation Rules · eclipse/capella Wiki · GitHub . Unfortunately, it is not very user-friendly for somebody inexperienced in these matters like me.

Is there any other way I can learn how to use EMF libraries and create custom validation rules? Maybe a sample or something that tells me what files I need to modify/add in my existing Capella installation.

Thanks in advance!