near a wall in general, put some watered down jam in the trap. however you could put it in the middle of the room at night and you'd probably get lucky, they're abit thick like that
we had a field mouse doing the rounds in a house i used to rent (lived in our wild never mown garden) while we were watching tv! use to watch it clamber over everything then go out the back door (which was usually open, if it was closed and the mouse was trapped we'd have to leave it ajar at night to let it out (poo's gave the game away).