There isn't a lot to say about this build, other than this: it's intended (mostly) to be played in Twisted Treeline. I don't enjoy playing Xin in Rift as much as in Treeline, so I can't vouch for how it will do in a longer Rift game.
Take a Doran's Shield and a Health Potion to start.
Getting Ghostblades allows you to level a turret in approximately no time whatsoever. So far, I haven't found an item I thought was better.
Depending on the situation, replacements for Phage/Mallet can be chosen, but the advantage it gives is extra health and help in chasing- though Lizard buff can replace that aspect.
Quints for health, I can't think of any others I'd like there. You need to be able to take some hits and flat health quints do exactly that.
Marks for armor pen, obvious. You're dealing physical damage.
Seals for dodge, can also be flat health. Depends on what you prefer or expect to play against.
The MR glyphs can also be cooldown but with Xin's Z/W and Ghostblade, I've found them unnecessary and I prefer the extra durability from MR.
DPS build, so I want 21 in offense. 21/0/9 is also an option, taking 15% increase in neutral buffs- you should always try to get lizard and dragon buff. You can also not take the Ghost mastery, choosing for Nimbleness instead- I just like having good Ghost.
Take your A/Q first. It is the skill that lets you hit as hard as you do and knockup is awesome. I see people taking Audacious Charge from time to time and believe me- it's absolute ****. It deals no damage, the slow is negligible and it just leaves you in a vulnerable position for the opponents to take advantage of the biggest weak point of Xin- no escape mechanism. :(
The leveling from there is pretty straightforward- you want charge next (yeah, I know, ironic) to harass and massively damage sloppy opponents that get careless. Then one Battle Cry to get your CD's off every time (plus, it's almost a free dagger without activation on level 1- sign me up). After you have all of those three skills, the most important skill is Crescent Sweep, then Three Talon Strike, then Battle Cry and max out Charge last.
You have two possible skill combos. R-A-Z is one of them when you get to initiate by just walking (stunned/snared opponent, stupid opponents) and you just keep your E up for two situations: damage opponents for the last chunk of health when blinded, or, of course, start the other skill combo when they try to run:
E-R-A-Z. Yeah, it's pretty similar, but I used to make the mistake of initiating every single fight with Audacious Charge- now I try and hope to keep it to chase them when they try to run.
When fighting against the lizard or dragon, only activate your Battle Cry only when you know you'll have killed it when the actives duration runs out. Don't use your ultimate, unless if you're halfway through it and need to go help out your mates.
Your goal is to get 2/0/x at some point in the game- usually if you can get there you can start dominating any non-tanks pretty quickly (and even tanks if given some time). Therefore, Shield and HealtPot to start. If your team is good for it, try to get First Blood- getting a decent First Blood gank (the kind before the minions) leads (to my experience) to a victory in 70% of the cases. This also means that you don't go into FB position with an average team against a bulky team.
If you start straggling (0/0/x at seven minutes or more deaths than kills), it's always a good idea to go for Avarice Blade before Brutalizer. The advantage is minor, but it's real. The item is also cheaper while still giving out a decent damage buff.
Once you get Ghostblade complete, you can destroy a turret in no time with Ghostblade and Z/W. The same goes for pretty much every building and champion. ^^
Later in the game (you have a Ghostblade, a Phage/other item) you need to try to spot the moments when you need to be helping out your allies in another lane, or putting some pressure in a lane on your own, to force the opponents to spread their attention and make them weaker. Also try to take down the dragon as fast and often as possible. Use every skill except for your ultimate, and if pressed for time, pop that R. It's important in Rift and in Treeline, it gets you an extra 10+% damage. Sounds worth it. And yet, I never see anyone taking it as soon as possible (for the record, I almost always Solo Treeline).
With Xin, bottom lane is your home. If asked to, you can solo, but not against every character. Especially Twitch, Teemo and Ashe can give you real trouble if faced one-on-one. Try to get bottom lane with a mate, leave the top lane to a character better suited for it- but keep an open mind.
There, this is my first attempt at making my own build. So far, it's served me very well. Compared to some other builds I've tried, you sacrifice a little bit of lategame power to be more powerful throughout the game. The "sacrifice" isn't even that relevant as your increased presence throughout the game will create obvious superiority in the endgame.
Feel free to leave constructive criticism- I'm not the best player but I am quite the decent player with Xin in Twisted Treeline. Try it out, have some fun.