Just taking a glance at the first few matches that you have I'm going to recommend a few things:
1: As a support main, I will say, DON'T play Lux support. Yes, she's a utility mid laner, but as a support, she's extremely niche. She's a mana whore and her ult offers 0 utility (which is BAD). However, in the case that you really want to play Lux support, makes sure you start with a gold income item like Spellthief's Edge, because Doran's Ring is practically useless on you. You only get 10 more AP from 
than from 
, you get no gold income from it and you'll never be able to proc the passive on Doran's because you shouldn't ever be CSing as a support.
2: You really need to watch your builds. You should probably be building 
on Leblanc in general, (usually as her first item after buying maybe 1 or 2 more rings) and you should've, in this particular game, rushed some MR before Xerath killed you 4 times in the first 10 minutes of the game.
I don’t know if you were just blindly buying recommended items or something, but 
should not be your go to item on Lux (nor should 
). Yes it does synergize well with her passive, but out of the laning phase especially, you shouldn’t ever be in range to proc your passive with an AA unless you’re really stupidly fed or you’re chasing someone solely because of how immobile Lux is. Other things: buy 
if you’re playing support. A lot more gold efficient than backing every time for wards. 1 sightstone = 10 green wards. You should be slapping down more than 25 wards a game. Vision is god.
You should be building AP items before completing boots early. Get base boots if you have extra gold. Upgrade if you enemy laner upgraded and he keeps outrunning you. Otherwise, boots are not of top priority. (Speaking specifically of this game where you should’ve been rushing Sightstone and support items/full AP instead of Sheen and boots.)
If you need help with builds, probuilds is always a good place to start. And as always, builds are suppose to be flexible and change accordingly depending on who you're up against.
3: I've never seen you play, but in general, you die WAY too much. Yes you should be trading in lane, but getting solo-killed 4 times by 
as someone as mobile as 
is really bad. Your KDAs are almost all in the negatives. That’s BAD. There is no way to improve your gameplay if you die more than you kill/assist every game. Positioning is extremely important. If you play Lux, you are extremely immobile. If you are caught out, you’re dead. Watch pros play if you struggle with this. Helps a lot.
Which also leads to my next point - don't play 
. This champ is very feast or famine. Either you get stupidly fed or you become the most useless member of the team. As of now, with the sole number of times you've died, I doubt getting stupidly fed is going to happen any time soon. (No offense really.)
4: You have to know what your opponent does. I could be playing 
against 
, who is my biggest counter, but if the Brand doesn't know what I do, I can still delete him off the map. I played a game once where I was 
up against 
. Being the bad person that I was, Kennen was one of the few champs I failed to glance over the skills for. So naturally when I try to trade, he gets his passive off on me, stuns me and then kills me. How to fix that? Watch the handy dandy skill vids in client of each champ while your waiting in queue. Even better, go to the wiki and read each champ over, including AP/AD scalings, cooldowns, etc. You don’t have to memorize everything, but if you were playing Lux vs a Yasuo, knowing that his wind wall has a base cooldown of around 20 seconds after it disappears will be extremely advantageous for you.
For example, in your game where you played Leblanc vs Xerath, here are how the two laners win vs each other:
As Xerath - Push the lane as hard as you can, especially since with your passive, you’ll go OoM a lot less when spamming your skills. Be careful about Leblanc’s placement when you cast your Q because you’re slowed and Leblanc can immediately dash into you and chunk your health with a W+Q+E combo. If she W’s, try to follow up with a stun. Best case scenario is to force Leblanc to waste her mana and skills trying to tower farm. Do not let Leblanc bully you. Poke her as hard as you can. WARD IN CASE OF GANKS.
As Leblanc - Watch when Xerath uses his skills. Most Xerath players will typically max their Q first to lower the cooldown and for more damage. That means that his W up until around level 7, takes 14 seconds and his E takes 13 seconds to go off cooldown. Bully Xerath when he uses his Q by distorting next to him while he’s charging up (to dodge the damage from the Q and a possible follow-up stun) and then follow up with a standard Q+E combo. This all needs to be super fast as Xerath can try to stun you and if he has W up, he can do a decent amount of damage to you as well. If you manage to dodge the stun, try to harass Xerath before his stun comes up again. If you’re too far away, you can also use W to gap close for a Q+E combo (only do this if you’re sure you can land the chains).
DON’T let Xerath poke you. Rush base boots early if dodging his skillshots is an issue. Play aggro, but watch his stun. Don’t let him push the wave, because a Leblanc forced to tower farm means a lost lane. If however you’re getting pushed very hard, ask your jungler for a gank. Xerath is extremely immobile and squishy, especially if you can bait out his stun so it goes on cooldown right as your jungler ganks. (BTW, in this matchup, you want to max W and get Morellonomicon ASAP for the cdr. You need the mobility to bully Xerath, that is your upper edge. If you’re still getting poked hard even with base boots, rush some MR.)
[[Also, why were you taking AD runes? Was it because you didn’t have an AP runepage yet?]]
5: CSing. Play customs and just CS for 20-30 minutes. Rinse and repeat. If you can’t even get a good CS score with no enemy laner, you’re definitely not going to get a good CS score with one. A good goal is around 100 CS at 10 minutes. (Pretty high standards, but if you can do it, very rewarding for your gameplay.)
[[And no, you are not reaching the CS numbers that you were claiming. 93 cs in 39 minutes is embarassing.]]
I really hope that helps with your gameplay a little! I know I'm only level 25, but I've queued up with plenty of silver and gold players and this is also their collective advice. My last piece of advice would be to play draft normals until you die a little less. Who cares about ranked?
