When we first adopted our boys, we let them have the spare room as their little base - their food, water and beds were left in there so they had somewhere to feel secure. We sat with them up there for hours at a time, just talking, letting them get used to the monotonous drone of our voices. Within a couple of days, they ventured out of their room and downstairs of their own accord, then scuttled back in again when they'd had enough.
This carried on for a couple more days, each little foray into the hooman world was longer and longer, until when they seemed comfortable enough we fetched their food and beds downstairs. That was it, they were fine all around the house then.