Heading North

I will need to be travelling to the US in the near future, and it will be my first time doing so as a residente permanente.

I understand there is a "checkout" procedure where you have to visit an immigration office and fill out a form prior to leaving Mexico.

Has anyone been through this recently? Particularly at CBX? The time I've gone through there on FMMs, nobody has ever checked anything on the Mexican side. As I recall, there isn't even a place for a checkpoint for people leaving Mexico. Not sure where I would go to take care of that.

Any information that will help me not screw up and / or not look like a moron is much appreciated.