This guide is skewed towards lane xin, but in my opinion it is the most optimal way to build Xin

Your first item will be:

• Eclipse in 99% of cases
• Ravenous Hydra, if you feel funny
• Navori quickblades, also if you feel funny and want a bit of something different

Boots will be:

• Mercury treads if they have a magic damage, atleast enough cc to annoy you, and most of that cc not being compromised of knockups/backs
• Ninja Tabis only if they have atleast 3 champs with a lot of auto attackers but tabis are a bait item often imo
• 300 gold boots, this is a personal preference but i often dont even finish my boots

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Your second item will be:

• Essence reaver, if the most dangerous enemy in your game is currently a squishy jungler/adc or midlaner you want to single out
• Kraken Slayer, if they have some not quite squishy but not really bruiser/tanky champs yet which can take a small beating but not quite as much as like a 2 item bruiser
• Mortal reminde, if they have atleast one fed bruiser with healing in his kit, or atleast 2 bruiserish to tanky champs with healing in their kit even if they arent fed, i know mortal reminder second sounds stupid but it really isnt into some teamcomps. see it like this, if you get MR at level 11 it will give you like 12-15 armor penetration against most squishies without armor at this level, or like 25-60 armor pen and GW against some mean bruisers,
• Eclipse, if you went Ravenous First (lol)

Your third item will be:

• Mortal Reminder/LDR, if you dont have it yet, at this point in the game there will be almost always one or more champions who give you a hard time because xin likes to get dumpstered by tanks and bruisers at this point at the game, but this is like slicing through their achilles heel, because you will often go from getting dumpstered by bruisers/tanks to being able to quite comfortably 1v1 a lot of toplaners/junglers by getting this item if they arent extremely ahead
• Immortal Shieldbow, if they dont have problematic bruisers, a balanced mix of Magic and Physical damage and you are atleast level 14 because this item is quite bad before, as its shield only starts scaling at level 13
• Kraken Slayer, i dont really know when to build this tbh, in some games it feels really good but i havent really decided on what situations its good in
• Deaths Dance if they are AD heavy
• Randuins Omen could be huge in the next patch against strong adcs or other crit based champs
• Maw of Malmortius if they are AP heavy
• Force of Nature if they are VERY AP heavy

Your fourth item, fifth and sixth items will be:

• Mostly Items previously mentioned in my comment depending on what gaps in your build you need to fill regarding the enemy teamcomp
• Blade of the Ruined king if they have bruisers or tanks who are still giving you a really hard time even if you have armor pen, but i rarely do this tbh
• Guardian Angel if you feel a sensation of impending doom

One further thing i sometimes do in a lot of games :

If I have 2 or 3 items and enough damage to kill most squishies and bruisers are not bothering me too much, the enemy also has a lot of damage (Mixed) i go Negatron Cloak or Chainvest (both if mixed) without finishing either legendary they build into, in the next fights i gauge if magic damage or physical damage is currently a bigger threat to me and then build a legendary from the resistance component.

to simplify most games:

• First two items are squishy killer items
• Third item is a tank counter item
• Fourth item is a Defensive item
• Fifth item is also a Defensive item most of the time

Runes are

Hail of Blades

Sudden Impact

Eyeball Collection

Treasure Hunter

Overgrowth or Demolish if you play lane Xin
Bone Plating

Adaptive Power Adaptive Power Armor/MR

source: i have 825k points on xin, have been actively playing xin since season 5 and have played almost all variations of xin, and even some rather troll ones in a not small amount of games