Xerath is a baller, he will smash faces if played correctly. To play xerath correctly you will need to follow this guide, a guide made by me TooAmazing. As you can see I'm too amazing for average guides. Keep reading and you will find youself pimp zapping through walls and single handedly winning games. I'm 1850 or something and have 67% win ratio with xerath in solo que, around 30 games played.

Here are the basics of xerath.
Long range
In built Magic Pen
High burst damage
Skillshot champion

Also looking at the top rated guide, the item build is wrong. You need damage early on and lots of it, rush deathcap.

YELLOW- Replenishment (IMPORTANT FOR PRE 6)
for early mana before your first back
BLUE- Clarity OR Force OR Potency
Clarity is because you run no mana regen or mana items mid/late game.
It is not essential when you have your two dorans.
You can run Force or Potency if are good at controlling the blue buff.

QUINTS-POTENCY
for early dominance, you can also think about 1 or 2 Move Speed Quints if you are running Potency BLUES
RED- INSIGHT
no brainer

21/0/9
Standard AP carry masterys.
The only variation I would use is changing out havoc for 2 points in butcher and 1 in brute force.
This will help beginners with last hitting.

Standard game goes as followed.

boots +3 pots,
dorans,
dorans,
Sorc boots,
rabadons,
void,
rylais,
sell Sorc boots and buy CDR boots.
sell doran get Hourglass
sell Droan get Scepter
if you are getting abused by magic damage buy the MR cape early and upgrade it later.

Max Q first
Max E second
W is maxed last
Take R when its up always.

R>Q>E>W

at level 4 you want to take your first point in W.

D- Ignite, Exhaust, Teleport, Surge, Revive(ballers only)
F-Flash (f is for flash no questions asked)

Take ignite if you feel you can kill your lane. The only reason you don't take ignite every time is because you don't go in range of the other carries in teamfights.
Take Exhaust aggainst a team with brusiers which will dive your team.
Take Teleport to teleport about the place.
Take Revive if you are a massive baller

Xerath can win games and is a strong laner. Can snowball well and is extremely underrated. I expect pro teams to start using him more.