In a nutshell....you don't. The older birds are highly likely to kill it. What you do is make a pen for the newbie to live in next to the older birds until it is close to the same size. By that time they should be more used to it and it will be big enough to protect itself better. It may still be a little hard at first since it's only one but with time it should be ok.
Alright is your hen broody at all? IF she is broody then its best to give the chick at night, if not broody its best you get a box and light and bedding and keep it warm yourself..