You need to open freebitco.in "Refer" section, where you will find your referral links, affiliate banners and forum signature code. Then you will need to promote your referral link - post it on social networks, forums (except bitcointalk, because here ref links is forbidden), blogs. Ask your neighbours, friends to join site via your ref link. It's not very easy to find many referrals on this site, because in bitcoin community, everyone knows about this faucet.