This achievement is linked to the completion of the first real "task" in Stacking.
The task is to get the three railway "managers" out of their special private bar/party area so they can meet with the striking workers and resolve the stop-work.
Take your doll - Charlie - and go from the platform area where you start towards an atrium area blocked by a gate. You will have to unlock the gate through the use of the doll with the key over his head.
From there, head up the stairs and through a few corridors and you will reach an atrium area with lots of dolls, an information booth, toilets, etc.
The area where the managers are located is up the stairs past the toilet area, on the right hand side. It is guarded by an overly officious security guard who will not let your doll into the room.
There are three ways to get past the security guard. The first, and easiest, is to find the maintenance worker doll nearby with the spanner above his head. Stack your doll inside him and do some maintenance work on the fan next to the door the guard is in front of.
This will allow you to enter the room where the party is being held. When you arrive, you will be identified as an "uninvited guest" and someone will start asking to see your invitation "lest everyone be thrown out of the party" ... which is of course exactly what you want.
Go to the doll asking you for your invitation and talk to him. The party will end and everyone will be thrown out - including the 3 railway managers.
Once that happens, stack all three managers inside each other and return back to the platform where the striking workers are. Talk to them and it will initiate a cut scene where the strike is resolved and a celebration of nutmeg is noted.
Your achievement should then pop.