Here is a list of some of the issues I found with this guide:
1. I would not put in a guide that you do not know anything about runes. You are supposed to be the "expert" on the matter since you are writing a guide for other people to follow.
2. In the item lists at the start you have Athene's Unholy Grail listed as the first item, but in the later discussion you mention getting Morellonomicon first. You should rectify that inconsistency. This inconsistency continues through to your in-game discussion, where you talk about Chalice of Harmony.
3. Building Sorcerer's Shoes for amazing early damage is wrong. You'd be better off to build straight AP over magic penetration on Xerath, since he has pretty good AP scaling. Upgrading the boots is good because besides the damage boost, it also provides you with a speed boost which is important on champions that are pretty immobile. But you should never go for Sorcerer's Shoes if you want to maximize your damage early on with Xerath.
4. Use better formatting for the Gameplay section (spacing between early mid late game specifically).

Hope this helps.

The Combo is Q-E-W-Q just to say and then R if you can kill him ;)

damn, someone read it :D thanks ^^

Just a friendly suggestion. Your combo should be E, W, Q. The stun makes your W easy to hit and since they are slowed makes your Q an auto hit also.