Hi! I'm Galatea and today I want to show you how to play Lux Mid.
This is what personally works for me but feel free to tell me other choices you like to use!

Why should you play Lux?

First of all, she's a very basic and simple champion but strong nonetheless. I highly recommend her if you just started to play Midlane or if you are interested in learning a fun but easy champion with little mechanics and lots of damage.
Lux is a siege mage, so she has a strong wave-clear and can delete the enemy fast thanks to her range. She's of early-mid game but it's very fragile/squishy.

Pros:

- Pretty easy and fun to play
- High AP damage
- Good crowd control
- Good early and kill potential
- Nice skins :p
- Annoying in lane
- Low cooldowns
- Heavy poke
- Strong wave-clear

Cons:

- Low mana
- Has a few high mana costs
- Squishy (can't take a lot of damage)
- Weak against assassins (If played incorrectly)
- Although easy you have to know how to play it right or you can easily lose

Lux fits well in almost every team. With her Q and W, she's helpful in team fights and with the damage and range of her R, she can kill dragons, barons, and enemies from afar.
But of course, there are certain situations where she's helpful more than others considering your team composition. Here are a few champions to consider but of course take them with a grant of salt since this is based on my personal experience.

Synergies:

She's a good ADC to pair with Lux thanks to her W. If an enemy is caught in it and you use your Q they are as good as dead. (Use this info if you're playing support too, they have one of the strongest laning phases in bot imo)

His knock-ups make ganks a nightmare for the enemy midlaner since you can easily R or Q without missing. Your W to help him out is a plus too.

Another good ADC to pair with Lux. With her W that slows enemies and reveals their location and her E that binds them, you can easily use your Q and R.

With his hook your Q + E combo is way easier to do without fail.

Counters:

He's annoying in general but with his shadows, he can easily dodge all your abilities (plus his damage is disgusting too)

Same as Zed, she can easily dodge what you have.

This is a hard matchup but not impossible at all to win. Vex is most likely going to take electrocute and biscuit delivery so it's going to be pretty annoying. She's also going to poke you and spam her Q and E.

He's pretty tanky and annoying. Can also easily dodge your abilities.

Here's what I personally take.

Arcane Comet:

With Arcane Comet harming a champion with an ability hurls a comet at their location dealing 30 - 100 adaptive damage, this allows you to poke the enemy very early on with your constant E, dealing extra damage is a plus to it too.

As stated before Lux struggles with mana, especially early on in the game. With ManaFlow Band Hitting enemy champions with a spell grants 25 maximum Mana, up to 250 Mana. And upon earning 250 Mana, Manaflow Band restores 1% of your missing Mana every 5 seconds. Pretty good if I say so myself!

Transcendence gives you ability haste when you level up to certain levels. This is very useful so you can abuse the enemy with your constant use of abilities (especially E and R)

With Scorch Your next ability hit sets champions on fire dealing 15 - 35 bonus magic damage after 1 second. This helps you deal more damage with your abilities, simple as that.

Basic attacks and damaging abilities deal 10 - 45 bonus true damage based on your level to slowed or immobilized enemy champions. This makes your Q + E or Q + R combo deal even more damage, giving you a lot of kill potential.

Reduces the cooldown of your R for each takedown you do, I think this one is pretty self-explainable.

You can also use as secondary rune:

Biscuit Delivery for a strong early (I go into more detail later)

Consuming a potion or biscuit immediately restores 30% of the health and mana. As you can see this rune plus the biscuits will help you have a strong early since you will have mana sustain and not run out of it so fast, being able to poke your enemy and farm to your heart's content (well...almost!)

Other options:

When talking about runes there are always going to be other choices that fit your playstyle. I'll help you find the perfect ones for you!

Dark Harvest:

Damaging a low-health champion deals bonus damage and harvests their soul, permanently increasing the damage of future procs. Base damage decreased. If your playstyle is more aggressive, dark harvest is a good choice. Make sure to only take it if you're sure you can get a lot of stacks on midlane or you're taking the risk of being potentially useless throughout the game(I would take it when I'm going against short-ranged champions for example). The damage is pretty good and it can make you have the best K/DA in the game.

I already explained this one.

Collect 2 Eyeballs from champion kills and 1 Eyeball from champion assists and ward takedowns, up to 20. Gain 0. 6 bonus Attack Damage or 1 Ability Power (Adaptive) per Eyeball, up to 12 bonus Attack Damage or 20 Ability Power (Adaptive). It's very useful and can help you balance since you're not taking sorcery.

I already explained this one.

Gain a Total Biscuit of Everlasting Will every 3 mins, until 12 min. Biscuits restore 15% of your missing health and mana. Helpful early in the game since your mana runs out fast. It also increases your mana cap by 40 mana permanently by consuming the biscuits.

It grants you 18 Summoner Spell haste and 10 Item Haste.

First Strike:

Amplifies your damage dealt to enemies by 10% for 3 seconds once you initiate champion combat, it also grants you gold afterwards. If an enemy initiates champion combat with you first you have. 25 seconds to engage in combat to activate the rune, otherwise, it goes on its full cooldown. It's an extremely poke-heavy rune and it's good for players that like being more aggressive as well. If you like to go in this rune is for you! I think it's worth saying that Lux is one of the champions that works best with first strike.

Slightly Magical Boots give you an additional +10 Movement Speed which is pretty useful considering how much you want to poke your enemy with this rune.

You can enter into debt to buy items. Debt can be up to 145 (+5 per minute), which is displayed as negative gold in your bank balance. When going first strike you want to take as much advantage of the inspiration rune and your gold as possible since you can go into lower debts.

I already explained this one.

Treasure Hunter grants additional gold to players the first time they collect a unique takedown. First strike, futures market, AND treasure hunter? You're going to be a gold beast in the best of cases! You gotta take full advantage of that!

You're gonna take this for most of your games:

Barrier is a self-targeted summoner spell that shields your champion from damage for a few seconds. As mentioned before, Lux is really squishy and can't take much damage so this spell really helps her along with her W. But ¿Why not ignite? First of all the range of ignite is 600 and all of Lux's spells outrange it pretty hard. It's true that Lux can work with all summoner spells but after level 6 you will most likely not ever use it again.

Flash is the standard and most used spell in the game. It helps you escape and engage.

You're gonna start with what most mages in the game start with: Doran's Ring and 2 Health Potion.

As for the general build, I'm gonna explain what you want to take and what effects it does!

For most matchups you want to take:

Luden's Tempest: Since it gives you 80 AP, 20 Ability Haste 600 Mana, and 6 Magic Penetration Stats. I think it must be the most used item on Lux due to how good and useful it is on her in general.

Archangel's Staff is personally an item I like to build on her. It gives you 60 Ability Power, 200 Health, and 500 Mana. I like it because as I said before, Lux is a squishy champion and has low mana so this item end's up being really useful. An important note is that archangel's staff transforms into Seraph's Embrace at 360 mana, which gives you 80 Ability Power, 250 Health, and 860 Mana.

Not gonna go into much detail in this one since it's also pretty self-explanatory. Sorcerer's Shoes is a very good item for ability power-based champions, as it allows you to ignore 18 points of your enemy's magic resistance.

And as for situational items:

You want to take Liandry's Anguish when the enemy team has a lot of tanks. It allows mages to burn through health and resistances and excel in longer fights. Damaging a target with damaging abilities causes enemies to burn for 15 (+2.5% AP) +1% max health magic damage per second for 4 seconds. Gain 5% magic penetration per second against burning targets (up to 25%).

Void staff is also an item you want to take if the enemy team has a lot of tanks. The 40% magic penetration from Void Staff becomes more effective as the target's Magic resistance increases.

I would take Banshee's veil if my opponents have a lot of CC. It grants a spell shield that blocks the next enemy's ability. It gives you 10 Ability Haste, 80 Ability Power, and 45 Magic Resistance.

UNIQUE Passive: Become Safeguarded and gain 75% damage reduction, lingering for 1.5 seconds after taking champion damage (40-second cooldown, resets upon taking damage from champions). Useful item when going against assassins. It also grants you 20 Ability Haste, 70 Ability Power, 250 Health, and 600 Mana. AND IT ALSO has a mythic passive which empowers each of your other legendary items with 1% bonus movement speed, and 8 ability power.

Passive: Illumination: Lux's damaging spells charge the target with energy for a few seconds. Lux's next attack ignites the energy, dealing bonus magic damage (depending on Lux's level) to the target.

Q: Light Binding: Lux releases a sphere of light that binds and deals damage to up to two enemy units.

W: Prismatic Barrier: Lux throws her wand and bends the light around any friendly target it touches, protecting them from enemy damage.

E: Lucent Singularity: Fires an anomaly of twisted light to an area, which slows nearby enemies. Lux can detonate it to damage enemies in the area of effect.

R: Final Spark: After gathering energy, Lux fires a beam of light that deals damage to all targets in the area. In addition, triggers Lux's passive ability and refreshes the Illumination debuff duration.

My tips:

In the early game, you want to hit your enemy with your E as much as possible. It deals a decent amount of damage and it annoys them to no end, making them leave the lane and not being able to farm at best.

Your combo is gonna be Q + E + R. Bind them with your Q, throw your E and finish them off with your R. It's so satisfying!

Be careful with your Q! It can save your life!

You want to max your E first, then Q, and last W. (Not adding the fact you have to max your R on levels 6, 11, and 16)

Try to use your W on two allies and not only one. Make the most of it!

Sometimes auto-attacking to get your passive can be too risky. I recommend you do it while being behind minions or when you have your Q ready.

A common mistake I see people do is not having map awareness. It's incredibly important for the use of your R so make sure to put wards.

Another mistake is instantly using your R. Although it has a low cooldown you still need to wait for an opportunity.

Some tips for landing your Q:

Use your E first. It slows the enemy and is generally easier to land something on a slowed enemy, right?
In the early stages of the game especially, search for semi-holes on minion waves to land your Q or you can Q a minion your enemy is behind. They usually don't expect it.
Hide in bushes and land it from there. The enemy can't see you so they won't try to avoid it.

As I said before, early in the game you want to poke your enemy with your E as much as possible. This can have three outcomes: Your enemy leaves to restore health and avoid getting killed, Your enemy will miss on the farm because they're trying to avoid your E, and you kill them because they stayed with low health.
Of course, there's also the possibility that you will be countered but Lux being such an easy champion I would say the chances of you missing your E are low.

Now, when do you want to be aggressive and when do you want to stay and farm?
You want to be aggressive if you have the dark harvest rune or the first strike rune. You also want to be aggressive if you have a short-ranged enemy in the lane.

Remember to place wards on both sides of the lane. You being the midlaner are in the center of the map and the jungler has two possible entries to gank you.

You want to farm with your E since it can damage a lot of minions and thanks to your passive you can out-farm the enemy pretty easily since it deals bonus damage when you auto-attack. If you want to ensure killing them you can use your Q on two minions that have low health but be sure to not waste too much mana.

When do you want to change your skill order?

There are only a few situations where you want to change the ability order I gave you. That's due to E being easier to land and helping you be more consistent in the lane. It also has a lower cooldown and it helps you wave-clear.
That said I can only imagine one situation where that would change.

If your team wants to invade.

If your team wants to invade then you should level up your Q no doubt. It will bind the enemy and if played right you will ensure first blood.

You want to avoid laning against Zed, Katarina, Yone, Yasuo and LeBlanc

This is because of their high mobility. They all can easily dodge your Q which can help you survive their high damage. It's not impossible to win against them but they will definitely give you a hard time.
If you go against them regardless I would build Crown of the Shattered Queen.

Team Fighting:

It's super important to land your Q. If you miss it you could potentially get your teammates killed. So don't rush and try to play strategically.
Protect your Jungler and go for the ADC.
Your jungler is important in general and ADC's are always squishy and easy to kill for mages/assassins. Since ADC's also deals the most damage and have the most attack speed it's beneficial to target them as well.
You want to position yourself in the middle or in the back.
It gives you security and also the opportunity to shield whoever is in the front or in the back.

Mid Game:

This is where Lux reaches her power spike. At this point, you want to already have your first item, which can be Luden's Tempest or Liandry's Torment depending on the situation I already explained.
You also want to pay attention to objectives such as Baron and Dragons, so always put wards on them. It can help you have vision and gives you the opportunity of stealing them with your R.

You want to always be with your team. Even if you buy Archangel's Staff, Lux is always going to be weak to some extent so make sure you don't give the enemy team the chance of taking you down!
For this phase of the game having 4 items should already be a given.
Make sure to choose them accordingly and don't play around! It can really choose the end of your game!

That's all for this guide! I'm always updating it and I'm constantly checking new ways to play Lux.

Thank you for reading!