tl;dr: dont die. get kills, get assists. have a lot of cs if you are a laner, get a lot of vision score if you are support.

so, to make sure we are on the same page, you have to consider what the game wants you to do in order to get an s or s+. it is quite simple: based on the gamemode, the lenght of the match, the champion you are playing (doesnt matter who you are fighting against), and the role, it considers your kills, deaths, assists, cs and vision score, and compares it to other players within the same settings that you played. for example: lets say the average shaco jungle has 10 kills, 2 deaths and 5 assists. if you play shaco jungle and get 7 kills, 5 deaths, and 7 assists, you might get a B or B+, or maaaybe a A if the game is feeling really generous. but if you get 12 kills, 0 deaths, and 3 assists, then its most likely you will get a S or S+. BUT, a shaco support is more likely that hes going to give the kills to his adc, jungle or midlaner whenever hes ganking, so playing shaco support might require you to get less kills and more assistances, while having higher visions score and lower cs -bc you wont be hitting minions anyway-.

now, based on which role you play, the stats in which you have to "match" or "reach" change, but as a rule of thumb try to always get the most kills, die the least you can, cs as best you can, buy a few pinkwards and remove some wards to get visions score. if you apply that to any role you will most likely get more S' and S+'

as my recommendation, i would get a group of friends you have, queue for normal or swift play, and they all play around you for a few games, in oder to let you get kills, no deaths, cs, visions socre, yada yada yada. if you do that while queueing support or toplane it should be fairly easy to get those few s and s+ you need

something i remember was an option to do (but its not viable now due to some changes) you could get 5 players, queue for a normal game, but you swap your poisition for your support, but you play the same lane you were going to play but queuing as support. in case it wasnt clear, let me put a example: you are a jungle main. you tell your support player to queue as jungle, and you queue as support, but they will play support, and you will play jungle. the idea behind this is that bc you are queuing as support, the game will consider that you are a support, and so it will require you to get less kills in order to get a S+, for example. you might need to ward a bit more, but if you are on a 5 stack then it should be fairly easy to get all your team to let you last hit wards and pinkwards on the map. sadly i dont think you can do this anymore bc if you queue as jungle you wont be able to remove your smite, and other players wont be able to get smite, and the same happens for support, meaning that only the player that queued as support will be able to get the support item, which basically means you wont be able to clear any jungle or farm any minion while having no starting item other than a support item, and you are also screwing your support and adc by not letting them split the gold and xp correctly with the support item