so im a S2 support main in EUW, currently yo-yoing between s2-g4 due to a lack of time to play and commit. sona was the first champ I played in league and has a very special place in my heart. I've been playing since around the end of season 8/ beginning of season 9 but have only began playing ranked at the end of season 9/beginning of this season.
op.gg link for anyone interested : https://euw.op.gg/summoner/userName=justint****16

sona has the delightful ability to support and have a large impact in games , whilst having a simple to learn , yet hard to master kit.
sona is a perfect choice for supports who would like to focus on improvements in other aspects such as warding skills , roaming and macro play, as well as helping you learn when to proc certain passives and abilities due to her very adaptive nature. if you enjoy supports with aggressive scaling and large teamfighting capabilities in the mid to late game , sona may be the perfect support for you.

power chord is an interesting passive which changes based on the last spell used , every third spell cast, her Q deals a short burst of ap damage, perfect for the level 1 fights against enemy support who disrespect you or attempt to score early damage.
sona's W chord works similar to an exhaust lowering enemy damage for a short period , this may be useful in lane where you see a potential fight breaking out and may help to gain an advantage by lowering the enemy adc's damage potential.
finally sona's E chord, a chard very useful for late game, ever caught out that enemy adc who's just flashed ? well now you can lower their movement speed , helping your team to move in and take them out quickly, this chord as well as the exhaust cord can also be used when backing away from teamfights and has the potential to save a low health team member who's attmempting to flee.

I'm sure with your interest in Sona, you've found the joke that she is essentially a caster minion early game and let me tell you now, that is absolutely a load of... well truth, imagine Sona as the Vayne of supports, early game she struggles, but once she finds her groove , there's no stopping her (well okay maybe a talon with his Leona that never wants you to breathe or throw out a heal).
Sona has the ability to turn potential losing teamfights into winning ones thanks to her passive aura from abilities and its large effect on allies, the shield from her W ability will hit all team members within the area, as well as healing sona and the lowest team member for some health which scales nicely with sona's AP.

be careful of intdividuals who build anti heal items. however if you force the enemy adc to build an executioners early, then congrats , you had so much presence and your heals were so annoying to deal with that they delayed building their items just to stop you stomping the lane with sustain what a proud support main you should be !

remember with sona. positioning is key to her survival, she is not a frontline tank (and if you believe mute girls with instruments make a good tank, then we need to talk about the jrpg obsession firs)Sona should aim to position herself at the back of teamfights, making it a risky task for that katarina or talon to kill you without getting killed themselves.
as for Sona's ult , crescendo is a powerful engage , disengage and team member saving spell , you could flash onto a lone adc who got caught out and throw out your ult whilst teammates close in , you could use it in a Teamfight to stun large groups whilst your teammates take out the damage dealers, and finally for those pesky assasins like Katarina's who yeet themselves into groups and delete, this ability is hard CC and will stop channels in their place , making them vulnerable (oh how the tables have turned mr/mrs Katarina main)

in some scenarios you may find yourself experiementing with other rune choices such as guardian sona for the in lane protection (sometimes reffered to as shield jesus sona with her insane healing potential ) and the beauty of Sona is that she is insanely versatile on runes , you want to proc aery constantly ? sure , did you like stealing items like a kleptomanic cat in season 9? sure why not , do you wanna go lich bane flash and stagnate power chord delete sona with a rune that harvests souls, sure why not. the rune choices I made are simply what I feel works best in the majority of games, so please experiment with runes, find something that fits your playstyle and go with it.

tldr: almost never
Sona's abilities scale with ap quite nicely, something that relic shield gives less of , the 25% mana regen on spelltheifs is also very important on sona as she has some serious early game mana issues ( hence the early tear). additionally that little boost in defensive stats from relic shield may still not be enough to keep you alive in team comps with heavy kill potential ( guardian sona may be a better choice ) and sometimes you just have to accept the death on botlane for now , you can make this up in the midgame!
10.15 notes: with the current mana sustaining buff to speltheifs edge I would suggest taking this support item on sona in pretty much every single lane, as it has made her early game potential much stronger once sona has access to a tear.

thank you for taking the time to read this guide , I aim to update it every patch in which changes to lanes may impact Sona ( including potential threats ) as the game changes. I do not claim to be a proplayer and as such there may be some interesting facts which I may not be aware of , please feel free to enlighten me on such things as I am always looking to improve. also this is my first guide written and im sure it has painfully bad formatting so im really sorry you had to sit through that !

good luck on the rift and I hope you can bring order to Bot lane through music ( see what I did there ? no ? okay i'll let myself out )