You need to build a mansion for your monsters to live in. Building can be done at any time as long as you have the building materials required.

Your mansion is made of rooms.
Rooms are made of a floor, walls and a door. Rooms that are missing one or more of these elements will not be complete and will XXXca fait quoi une piece pas complete?
The first piece of furniture placed in a room determines the room type. A room can only be of one type. To change a room type, you must recycle all the furniture in the room associated with that type.
After a room type has been assigned, only furniture belonging to that room type can be build in the room, with the exception of furniture that can be used in all rooms. For example, you will not be able to build a bed in the kitchen, or a fridge in the home office, but storage cabinets or traps can be build in every room.

This is the first room you should build as your monsters will need to sleep. monsters will fall asleep on the ground whenever needed, but it's better to let them sleep in a bedroom
To create a bedroom, add a bed in a finished room. There are different types of bed and and some monsters prefer sleeping in a particular type of bed.
Only furniture from the Bedroom menu can be added in this room.
The monsters will automatically pick the bed that is best for them - just make sure you build enough beds and they will sort everything out.
To start with, place a generic straw bed that is suitable for all the monsters. Later you will be able to upgrade the beds and add more furniture to make the room more pleasant. To make the monsters really happy you can give each of them their separate bedroom; and remember that a luxurious guest bedroom will attract even more victims!

The second room you need to build is a home office where where your monsters can send letters and print invitations to lure victims and kill them.
To create a home office, add a writing desk in a finished room.
Only furniture from the Home Office menu can be added in this room.
A monster will then need to be assigned the job of writing letters. See the XXX JOBS XXX section.
Later you can add other communication tools to this room to be more efficient at attracting victims.

The third room you need to build is a kitchen so that the monsters have a way of processing the dead victims to prevent the food from spoiling.
To create a kitchen, place a butcher's block in a finished room.
Only furniture from the Kitchen menu can be added in this room.
Monsters can then use the butcher's block to process the dead victims and the fridge to store the meat.
Later, you can add a fridge or other kitchen apparatus to process more food, more quickly, more efficiently.

The fourth room you need to build is a research lab to develop the technology to build more complex equipment like a smoker, etc.
To create a research lab, place a research bench in a finished room.
Only furniture from the Research Lab menu can be added in this room.
A monster will then need to be assigned the job of working in the lab to research various technologies.
Later you can add more powerful research devices in this room.