A lil' bit of attak damage makes your early harass with basic attacks and Q hurt more.

A big amount of armor lets you shrug off enemy carry harassment more easily.

Magic Resistance is important against magic damage supports (or even carries like Ez or Varus) and junglers.

2 gold quints lets you avoid the need to waste your early money on a gold item instead of the Sightstone.

As the third quint, you can chose between another gold quint (to reach 5g per 10), a flat health quint ( which I used for a long time) or a movespeed quint (to catch up better and reduce the impact of not buying boots)

We take increased hp, resistances and reduced damage from basic attacks (enemy champions as well as towers) from the defense tree. This, along with the runes and W skill, makes Leona damn beefy early on and allows you to harass efficiently.
The utility tree gives us mana regen (so we won't need philo stone for that) increased gold again (alongside our runes to again, avoid an early gold item), a free ward, free hp/mana heal and shorter CD for our summoner spells and activated items. All around, this boosts our support role and allows us to focus on tankin and CC spamming.

Shield of Daybreak:
Q at lvl 1, with a 1,25 seconds targeted stun, this ability is great! Also it triggers and counts as an auto-attack, meaning it actually deals the most damage of your 3 abilites at lvl 1.
The fact that Q triggers an autoattack makes you a great ward clearer.
Just stand at the edge of your lanebrush and wait for the enemy support to ward it.
Leona will automatically attack it, after the hit you immediatly activate Q for the second hit and if your attack speed is high enough (or your carry pays attention) the ward will be hit for a third time, wasting your enemies gold (your atk speed should be high enough between lvl 1-3, depending on your reaction timeaswell as rune/mastery layout.

Eclipse:
big AoE range, nice base damage and high resistance increase - always activate this when engaging, also max it first for increased resistance bonus, this allows you to focus on hp items while maintaining good resistances.

Zenith Blade:
you can use it to help your carry push the lane (thanks to your passive), other than that, this is your main initiation tool and allows you to ignore buying boots early on thanks to short animation and great range.

Solar Flare:
Epic cast range, epic AoE range, strong slow and 1,5 seconds stun in the center.

When you can get close to your enemy, your combo goes
W+E -> Q -> R
this grants you resistance while unleashing your full CC, and easy hit of your ult thanks to Q stun and a low chance of not hitting with your W for longer resistance bonus

when you can't get close to your enemy
R -> W+E -> Q
your ult stun can get you enough time to get in E range of your target, problem with this combo is that you could miss your ult, which is bad
this combo is also useful when your enemy frequently dodges your E (e.g. Vayne, Ez) since you can cast it from the brush and people will anticipate your E, not your ult

Get a feeling for your CC times, having Q and R hit at the same time would waste a lot of CC.
the perfect combo unleashes 0.5s snare + 1,25s stun + 1,5s stun on your enemy, that makes 3,25s lockdown where the enemy carry can't fight back (especially during laning).
Keep in mind though that your enemy could flash away if you give him the chance between your stuns.

Starting Items:

HP Pot: 150hp over 15s = 10hp every sec.
price 35g => you get 4,29 hp healed per 1g

Rejuvenation Bead: 5hp over 5s = 1hp every sec.

price 180g => as 1g in hp pots is worth 4,29hp the bead needs to heal 772,2hp to be gold efficient through healing alone, this would takeabout 13 minutes.

If you plan on selling the bead (worth 126g) then
180-126=54g => 54x4,29=232hp
the bead only needs to heal 232hp to be gold efficient, which takes it roughly 4 minutes.

Sight Wards are a must have, ward either tri-bush or drake after ~3minutes or when you push.

Always carry atleast 1 vision ward, use it to either clean a path for your jungler or simply to **** the enemy support's warding (some supports stop warding when you do this, bad junglers get scared of ganking too)

Also eat the cookie as soon as you used a skill and took some damage.


As Fast As Possible:

rush the Sightstone, it's more useful than philo stone.

You pay 475g for the hp (perfectly fine as it is the ruby crystal price) and another 475g for the effect.

As every Sightward set would cost you 75g, this item turns gold efficient after having used it for 7 wards. If you always use it to set 2 wards at a time, then you achieve this mark after only 12 minutes after buying.

Ruby Sightstone is similar, only that it costs 125g more for the recipe, but it enables you to set 3 wards at a time, therefor turning gold efficient even faster.
Also the hp makes you tanky, wards and philo don't.


Core Build:

Objective 1: always carry 2 sightwards and a vision ward

Objective 2: obtain Sightstone, carry 1 sightward if possible, always carry 2 vision wards

Objective 3: obtain Ruby Sightstone, carry 1 sightward (optional), always carry 2 vision wards (or oracle if you can afford it and the enemy team wards a lot)

Objective 4: if you need movespeed, buy boots - if not, go for aegis, carry 1 sightward (optional), always carry 2 vision wards (or oracle)


Late Game:

This is what your End Game build should look like - atleast.
You can switch out Solari for Shurelia's or Michael's if needed, but you definitely should have 1 active support item.
Upgrade boots according to enemy team (fed cait and yi ? ninja tabi - amumu and blitz ? merc threads - you are ahead and your team needs initiates ? mobility boots - the enemy team only has slows ? boots of swiftness )

If you can afford more items than this - buy smart !
more active items to assist - move speed to catch - tankyness to engage - CD reduction for more CC spam, whatever your team needs.