Hello guys!
Very short introduction to a very short Leona guide.

I am SUΡΡΟRΤ, a Master EUW Leona main who is also a mentor of the official r/summonerschool discord.

OPGG: https://www.op.gg/summoners/euw/SU%CE%A1%CE%A1%CE%9FR%CE%A4

Inspiration:

Glacial is the best rune for Leona because of the damage reduction of 15% on enemies.
It also allows you to hold your Q after hitting E because your ally can already get in range of the enemy due to the slow.

Hextech flash is a must have against ranged characters. If you can trade flashes with the enemy ADC/Support always do! You have another surprise for them in 17 seconds :)

Minion demat in order to push faster in order to increase your roam timers. If you are below diamond I would recommend taking cookies instead.
Little tip: You also get the bonus mana from cookies if you sell them.

always Cosmic insight

Resolve:

Second wind is by far better than Bone plating since a single attack by the enemy Support or ADC can proc your Bone plating and it has a long cooldown. Only take Bone plating against a melee Support + a shortranged ADC.

Unflinching is good in order to maximise your cc in teamfights and gives you slowresist. Slows are very effective against Leona and that bonus slowresist is really good on her. If the enemies have almost or no cc you should take Font of Life instead of Unflinching.

Always rush boots first!

Boots are extremly important on Leona as you are required to roam a lot and you want to have as much presence on the map as you can.

Boots of swiftness are the best boots for leona since they give the most movement speed. Mobility boots are not as good as swifties since you lose the movement speed buff from mobility boots after you engaged on an enemy. You want to keep following the opponent after your initial engage so mobility boots are not really an option for Leona.

If you want to play for your lane and you are dominating botlane you can go plated steelcaps for some extra tankiness to go for riskier plays and prevent giving shutdowns.

In rare cases where the enemies are a team of almost all full tanks you can go lucidity boots in order to maximise your cc and have your W up more often in order to be tankier.

Why we don't take mercury treads: If enemies are throwing their CC onto you, that is a freebie and your backline should be able to move freely and kill the enemies more easily. But in most cases you are not the target of cc abilities and therefore don't need the extra tenacity from mercuries. Lucidity boots make you tankier against AP heavy teams than mercury treads do, since it allows your W to stay up more often.

Your second item should either be Evenshroud or Solari.

You should go Solari if the enemies have champs like Karthus, Diana, Talon etc.
Champions that can hit multiple of your allies and/or burst your allies should be less deadly for your team if you go Solari. If there is only a single enemy champ that can burst a single ally (Zed for example) it is not worth to buy Solari.

Pretty simple:

If your someone on your team is super fed you go knights vow, if it is someone from the enemy team you go anathemas chains.

Health is worth much more than resistances on Leona since she already has resistances from her W. Someone with 5k HP and no resistances or someone with 300 Resistances and 500 HP is not as tanky as someone who has 3k HP and 100 resistances :)

Anathemas chains/Knight's Vow: Already explained.

Randuins Omen: The enemies have a lot of physical or crit damage.

Frozen heart: The enemies have multiple ADCs or a lot of onhit damage.

Force of Nature: The enemies have a lot of magic damage.

Abyssal Mask: The enemies have a lot of magic damage + Your own allies have a lot of magic damage.