Xin Zhao The Seneschal of Demacia can be played as a Tank or Meele DPS but the build I choosed is a midway of this two variants
I'm Skeec and this is my first guide so lay back and enjoy!

This Guide is still not finished so please do not downvote for structure or missing data this will be added later and I apologize for my english I'll take a look on it later

• greater quintessence of desolation: Armor Penetration
• Greater Quintessence of Health: Extra Health increases his weak early
  Personal preferences may vary but I use the Armor Penetration
  
• greater mark of desolation: Nuff said Armor Penetration
  
• Greater Glyph of Scaling Magic Resist: Magic Resistance for more tankiness
  
• Greater Seal of Armor: Armor for more tankiness

Okay I recommend Armor Penetration because his Ultimate has to deal True Damage in order to be effective and his Three Talon Strike is Physical too so self-explanatory

Quintessences can also be changed in order to be more tanky. I would recommend HP then but you can use Armor or Mresi too.

Magic Resistance per Lvl is only because Abtilitys need to be skilled means higher lvl results in higher Damage so the Resistance is needed more later when enemys abilitys hit Lvl 5

Ignite : Most common Offensive Summoner spell! See here Exhaust : Is a really strong Offensive Choice because you can also use it for defensiv uses and it does reduce enemys armor by 10 if improved in masteries

space

Flash: Most common Defensive Summoner spell! See here Ghost: Second Defensive Choice but as always Summoner spells are personal preference based so choose them like you are used to

space

• Challenge: Xin Zhao challenges his target with his basic attacks and Audacious Charge, reducing its armor by 15% for 3 seconds. Only one target can be challenged at a given time.
  
  
• Three Talon Strike (Q): (Active): Xin Zhao's next 3 attacks deal bonus damage and reduce his other abilities' cooldowns by 1 second each. The final strike also knocks the target into the air.
  Cost: 30 mana Cooldown: 9 / 8 / 7 / 6 / 5 seconds
  Physical Damage: 15 / 30 / 45 / 60 / 75 (+1.2 per attack damage)
  
  
• Battle Cry (W): (Passive): Xin Zhao heals himself on every third basic attack.
  (Active): Xin Zhao unleashes a battle cry, increasing his attack speed for 5 seconds.
  Cost: 40 mana Cooldown: 16 / 15 / 14 / 13 / 12 seconds
  Health Restored: 26 / 32 / 38 / 44 / 50 (+0.7 per ability power)
  Bonus Attack Speed: 40 / 50 / 60 / 70 / 80 %
  
  
• Audacious Charge (E): (Active): Xin Zhao charges and challenges an enemy. The charge deals magic damage to all nearby enemies and slows them for 2 seconds.
  Cost: 60 mana
  Range: 600
  Radius: 225
  Cooldown: 13 / 12 / 11 / 10 / 9 seconds
  Magic Damage: 70 / 115 / 160 / 205 / 250 (+0.6 per ability power)
  Slow: 25 / 30 / 35 / 40 / 45 %
  
  
• Crescent Sweep (R): (Active): Xin Zhao unleashes a sweep around him that deals physical damage to all nearby enemies equal to a base amount plus 15% their current health and knocks them back. Xin Zhao gains bonus armor and magic resistance for 6 seconds for each champion hit.
  If a challenged target is hit by the sweep, it is unaffected by the knockback.
  Cost: 100 mana
  Radius: 375
  Cooldown: 100 / 90 / 80 seconds
  Base Damage: 125 / 225 / 325 (+1.0 per bonus attack damage)
  Bonus Armor and Magic Resist: 15 / 20 / 25 per champion hit

Attention:
More in-depth ability usage and videos will be added later so stay tuned ;)

Your first decision: Which starting item?

In order to start effective you should decide toward your Opponent and your Core Item
Lets say you got Rumble as laning Opponent

• Magic Resistance would be smart because Rumbles abilities do magic damage

Probably Null-Magic Mantle + 2 Health Potion

• Wait you will not get Mercury's Treads? Because you fell better to get Berserker's Greaves

Conclusion: Null-Magic Mantle is not viable

Which alternatives do we have?

• Boots + 4x Health Potion ( to get 4 you need to stay in pool for a few seconds )
• Cloth Armor + 5x Health Potion

The amount of Health Potion is crucial for the lane survivability
So Boots help to dodge his skill-shots, Cloth Armor provides one more Health Potion and loweres the damage of Rumbles autoattacks and the minion damage you take, but it also forces you to buy Ninja Tabi because we do not buy Madred's Razors

• why not Vampiric Scepter? Because you will run around with it the whole game till you get Bloodthirster if you even will, but there is Wriggle's Lantern okay I'm not a fan of this Item but it's still there so you can go the way of Wriggles I won't forbid it

Ok the step with the item to start of should be clarified

Let's talk about boots

Defensive Options

• Mercury's Treads If enemys got a lot of CC Note: Surpresses are NOT reduced by Tenacity
• Ninja Tabi If enemys got crazy late game AutoAttacker like Tryndamere

Offensive Options

• Berserker's Greaves Standard if everything runs okay and you want to get more DPS

Building toward Core Items

If you feel too squishy you can move forward Phage first, but I recommend to rush Black Cleaver as 1st Core Item after T2 boots!
Black Cleaver synergizes very well with Xin Zhaos Q because it procs them all at once kinda yeah and noone else needs Attack Speed so much as Xin does!

In order to get the tanky aspect I would like to recommend this guide
There is an example given to Xin Zhao it is a little bit old, but it still works
To briefly inform you the guide says initially Xin Zhao got enough Resistances due his Ultimate so build Health Points (rare but possible if you hit 5 enemys your ult adds up to 125 Magic Resistance AND Armor for 6 seconds on lvl 3)
My favourite Item that offers a lot of Health is Frozen Mallet
So we do not only build it because of its health it delivers a great not to say one of the best slows ingame and our loved Attack Damage so this item is so bought :)

Beyond Core Items

Normally the game is like already won at this phase, but if its not you have to make the next decision!

Offensive Options:

• Infinity Edge Quit the major DPS item
• Phantom Dancer If you need movement speed over DPS to chase/escape
• Bloodthirster Pure AD scales good with your abilities yeah and tons of life-steal
• Last Whisper If the enemys are heavy armor monsters
• Madred's Bloodrazor If the enemys got Volibear or Cho'Gath for instance with high health but not as high magic resistance
• Youmuu's Ghostblade Not that popular but still a good choice

Midway Options:

• Maw of Malmortius If you feel to get more Magic Resistance
• Atma's Impaler If you feel to get more Armor
• Wit's End It deals magic damage and I don't really like it because just a bit of magic resist will make its damage to under the roof if you know what I mean
• Sunfire Aegis Same as in Wit's End case

Defenisve Options:
Okay there are many. I'll just list a few

• Frozen Heart It's the best Armor Item beside Thornmail
• Randuin's Omen This item is great against Vayne and Corki for instance because they deal true damage and Randuin's Omen loweres their Attack Speed Frozen Heart provides a similar effect
• Banshee's Veil I would say it is the best Item against magic opponents
• Force of Nature Against high magic damage opponents furthermore it provides movement speed and major health regeneration
• Quicksilver Sash Common for Ad Carrys to get out of s*** free card but this also works on Xin Zhao when your enemys get to know you deal the main damage you will always suffer under Exhaust so this could solve this disease
• Guardian Angel The quit most popular "last round off item" against mixed damage and a 2nd Life *~*

• Thanks to JhoiJhoi! Her epic guide can be found here! And her Template :)
• Thanks to Prant and DiffTheEnder for their epic guides! I love "A different view" <3
• Thanks to Cormag for his Health and Resistances. An Optimization Guide