Before it arrives
Check the delivered footprint against the room, the doorway and the stair turn. A cabin arrives as a freight item and usually needs two people to move.
The electrical circuit
Most home saunas need a dedicated circuit. Confirm the voltage and amperage in the manual against your panel before the cabin is assembled, and have an electrician fit the circuit if you are not certain.
Floor and ventilation
A level, dry floor. Leave the vent clear and do not box the cabin into a corner without airflow.
First heat-up
Run the sauna empty for the first cycle to burn off manufacturing residue, then ventilate the room before first use.