Hello! I'm not a high elo player but I've played a lot of Xerath so I can say i have some experience with him. This is just a guide for players who wants to learn this champ and maybe get some inspiration to play him.
This is a guide that is more in the teamfighting aspect and not the laning in particular.

This is my very first guide so it's not going to be the best in quality compared to others. :)

Pros:
High range that gives the upper hand against most opponents.
High poke potential that makes your team get the advantage of the teamfight aswell as map control.
Great AOE spells.
Awesome passive that lets you sustain your mana throughout the game.
Great waveclear.
Safe pick in most situations.
If you fall behind you can easily get back in the game.
Fun to play because of his kit.

Cons:
Low mobility.
All spells are skillshots.
High skillcap, therefor hard to master.
Ultimate that takes some practice to get used to.

It's a must have on Xerath because his lack of mobility.

This is your lifesavior. It gives health aswell as movement speed. You can switch to barrier or exhaust if you are up against a burst champion.

I don't like to use this on Xerath. The main reason is that you need to be close to use it. With the range Xerath's kit has, you don't want to be that close at all time.

Here is where Xerath gets to shine.
Before the teamfight even begins you can poke the enemy team down so you and your team can go for objectives while they recover.
You can even sometimes kill one of their carries even before the fight starts.
In teamfights you always have to think of standing behind your team. It's because of your lack of mobility and also the awesome range you got to take serious amount of damage.
Use your ultimate to either kill an important target before the fight or to clean up.
REMEMBER!
Do NOT ult if the enemy team can get close to you. You have the sickest range and are rooted so stay FAR behind when you ult.

Luden's Echo: Great item that gives you extra damage to your poking.

Rylai's Crystal Scepter: The passive on this item is so good on Xerath. It is so much easier to land your skillshots when you have a slow on the enemy.

Morellonomicon: CD Reduction with high AP. The passive makes it easier to pick up kills as heal wont do as much.

Void Staff: Great item. It lets you get nice damage on the frontline and melts their backline. This item buffs Xerath's lategame so much.

Rabadon's Deathcap: Sick AP that gives Xerath a huge power buff in the game.

Sorcerer's Shoes: A must buy. Great magic penetration that you just can't ignore.

As I said this is not the best or the most complicated guide you can find. I just want people to see how I play Xerath and maybe can learn something from this guide.

If you think this champion is hard to play in the beginning, just keep playing him. It will reward you when you can play this champion!