3. What to do if the villager has already moved out
Unfortunately, this will not guarantee a successful adoption. This will only work if the adoptee has not gone online whatsoever.
Simply put, the adopter must be the first person to go online with the adoptee. Via a hidden "town pool" (explained at the bottom of this post) it is still possible for the villager to move in.
This method is extremely unreliable, as you can get any villager currently existing in the pool, and we don't recommend you use this method unless if you're absolutely desperate for the character.