Don't go for
[*] Warmong's and Thormail than
[*] Sunfire Cape and Randuin's Omen.
The last two have better passivie and antive uniques that help your champs...Thormail passive is not needed. It is usefull if enemy's adc is hitting you, if he does....the fight is over whatever armor item you have.
Sunfire and Randuin's gives you more armor and more health than the other combination.

[b]Don't prefer Dead man's plate than Randuin's Omen.[/b]
Remember...you are a support. You want enemy's team slowed so that they cannot reach your adc or the mider. You are not the one who hunts for kill. If you do take it, change your boots from Alacrity to Captain.

[b]Mikael's Crucible or Solari?[/b]
Are you fast? Mikael's is an item that you mnust use fast enough! If you can, then use it against player's as Morgana, Braum, Annie, Rammus, Fiddle, Shen. Solari, is more practical and gives you health...remember you are a tank and support.

Spirit Visage or Banshee's veil? Why not go for Mikael's amd Solari together?
You are a tank! 70 magic resist and 500 health...far tankier item than Solari and Mikael's for late gameplay...(with your shield +20% is a thing to consider). I would prefer Banshee's veil is enemy champ's have one cast that may change your game. Like Morgana's Root or Blitzcrank's Grab...or even Lux ulti. But what about champ's with constant ap damage like chogath spikes, or Katarina etc? Spirit Visage helps you to stay in game longer, as you already have health regenaration from Solari and the upgrated sightstone.

Vision ward...in case of red side, then obviously you know its use....in case of blue side...it protects the blue buff. It's called teamwork!

Sweeping lens...you are the one who uses most of the vision analogicly...Sometimes you can feel it, if they see you, they kill you...

Its enemy champ in bot lanes must be engaged in a diffrent way. Not?
If you play the same way against all champ's then you are probably doing it wrong. In the chapte "VS champions", some examples against some support champ are given.

But what about the enemy's adc?
Some Champs like Vayne, Lucian, Ezrael, Draven and can dodge your ulti. Use your ulti when they have used their abilities that can dodge your's. Keep in mind that it's best that your adc have more range than the enemy's as you are melee and cannot harass them easily.

leona is the Queen of stun,
use your blade and stun when your adc is too far from the stunned champ and consider youself harassed if no dead. Count the risk...and the probability that your adc will hit the target when you stun.

Always Stun and Blade together?
NO!. Sometimes it is better not to rush the stun. Your adc may sleep of be far, it maybe also rooted or stuned also. Always youe eyes on your adc and decide when its is time for a stun using your brain!

Sometimes you may have to stun on champ just to give the kill to your team. Sometimes you may even have to ulti only one champ...
Things to consider.
If you don't have too many wards in the area....it's better to keep your ulti and not use it for one champ as you don't know where the others are! If the player can dodge it, it is even worse. Exception if this one player is the fed one and cannot dodge.
May your team doesn't like it, but it may save the day.
Ward like crazy...you are the one who stuns, remember? if you see, you can stun, if you stun, your team may have a free kill. Its more than common practice for leona.
Example. Blade (0,5 sec), stun (1,25 sec), ulti (1,5 sec), Activate Randuin's (35% less move), Your team has 3,25 secs to came to you...and even if they need to hunt...your omen supports for an easy kill.

Moreover,
In teamfights sometimes its better to use your Q to protect your adc or mider.
Consider stoping an enemy's ulti....(katarina, just easy)