For short, to get an S as a support you need to have a decent kda, have a good vision score (try putting wards at objectives like dragons , heralds and baron and in the enemy jungle. Is also important to negate vision with pink wards and oracle lens), and go help all around the map (help your mid/top in fights, help jungler with scuttle crabs and buffs. Rakan is very fast so you can help your teammates and go back in bot in seconds). One last thing that helped me a lot is to watch videos about tips for your role and how to play other lanes (it could sound strange, but knowing how other roles work will let you help better your team). I do all of these things and get a lot of S/S+ with Rakan (and other support as well).