Early game:
The most important thing is to not get caught, when you got that mastered you can try and safely use your Love-Tap(Passive) for a little poke and wave control. Try and punish them not respecting your base dmg as a marksman

10-13 mins (Ardent censer purchase):
Now you can actually fight and make sure your adc knows that too. You are strong, intepentant woman that you should have about 1.5 attack speed (with W active) and 25 on-hit dmg. As a support that is a lot of dps.

After 20-ish min to End of game:
Here you should be using your slows and your ability to move fast af to catch enemies out of position (Remember to do it with someone. You're very weak alone). If you already have your Rylai's you can start to fight 5v5 or 5v4 bc of your picks. Try and spread out your slows from E and Rylai's with Q and Ult.
You should be able to get 'some' autos in, but rather stay safe and keep your W passive movement speed for as long as possible.
This build and play style should give your team a dps advantage make you able to win almost every 5v5 teamfight, when not caught out by burst.

Somehow i wasn't able to add runes when i made this (will try and edit it if possible). Sadly that doesn't spare you from getting to know this very unusual rune page ;)

9x Hybrid pen marks
9x Gold/sec seals
9x Flat cooldown-reduction glyphs (Reaching 7.5% cdr)
2x Gold/sec quints
1x Flat cooldown-reduction quint (Reaching 10% from full runepage)

If someone knows why i can't add runes as intended please tell me :^)

Make it rain (E) should be maxed 1st for shorter cooldown and for that extra thicc poke dmg. Remember even though her E is only raining bullets for 2 secs it enough for you almost constantly apply ardent censer to your partner and yourself.

Strut(W)'s movement speed and activatable attack speed combined with ardent censer's attack speed/on-hit that now also applies to you (Patch 7.19) gives you quite the huge dps boost

Double-up(Q) doesn't make the great difference as MF supp since you shouldn't be killing minions to make its bounce stronger plus the ap ratio is quite bad

Bullet time(R) is mostly used for getting the last shots to finish off your enemies in lane or ending your round of spells in a fight. When you get to purchase Rylai's it is also very effective at getting out slows and therefore proccing your keystone 'Bond of Stone'