How to create object's routing slots for interactions?

Hello, this is something that I've been struggling with since a very long time and I never understood how it worked.
I'm attempting to add a "Routing Slot" to an object where sims would start the interaction after routing to that specific slot, I've looked at the "Photography Studio" object that came with get to work & it has multiple routing slots in it but I couldn't figure out how to add it/make it work for my own object. 
The regular constrains that are added to some interactions are not very precise and they mostly don't work for what I'm trying to achieve. 
Is there any tutorials on how to implement routing slot based interactions? that would be much appreciated. 
Thank you very much