For Charlie we just got him a really comfy bed, made sure he had a warm blanket, and a teddy bear. Then we just closed our door. He whined for a bit the first few nights, but by the 4th night he had learned "go to bed" and off he went ! It was hard those first nights when he whined, but we just let him do his thing. I think the first night he whined for about an hour and then he fell asleep
Good luck !