this is started at first day xerath came out. he is now my favorite champ after a few games because finally i found a champ perfect for my playstyle. for now i will recommend a few things and hopefully will make a full in depth guide in a week or 2, so come visit to keep updated.
i also predict a little nerf because some of the features of xerath are just way too easy to own with.

so for runes i recommend magic pen reds, mana regen yellows, cd reduction per lvl blue and flat AP quints.

magic pens are MUST on any ap based champ

the mana regen runes will make clarity futile as this champion is not mana hungry if properly used, i maybe used clarity once or twice in about 10 games played with him, so take it so you replace clarity by another summoner and still be able to harass well

cd reductions is what will make you useful in team fights as multiple burst in short time will help your team. it makes you able to stun twice in the same teamfight

flat ap quints are for the ap boost you need to dominate laning earlygame so you can harass and farm efficiently.

for masteries i use 9/0/21
the important ones are the magic pen in offence, the 9% cd reductions the rest is your choice

start with doran ring if you duo lane, boots if you solo mid, i do not recommend solo top with xerath yet

first thing you want is to get your sorcerers boots and amplifiying tome at first call back minimum

if you are having very hard time, get a gold/10 item that suit your situation asap

first major item should be ryleys scepter, gives survivability, the ap, nice passive, AND little armor due to your passive.

team fights should start now and cd will be more than welcome, morello's tome is next to build

deathcap for a huge ap boost, now enemies will fear you, you will be targeted more often

build abyssal scepter as it completes well your survivability and gives you more ap and magic pen. magic pen passive of this items may not apply if you shoot from very long range

sell your doran ring and get a situational item, if they are stacking mr, get void staff, if you feel agresive, time to get lichbane and do crazy combos, survivability? get zonia hourglass

if game goes forever, get the buffs every 4 min

priority:

ultimate
your q is main source of harass and damage,level it as fast as you can
your w is what it makes you a ''mortar'' so be sure you pick this at lvl 2 and level it up only if you feel the need for a shorter cd, i start leveling this up after when team fights occur
your e should never be used to deal damage, only if all other spells are on cd and your target is very very low health and you need the finishing hit. otherwise, this spell is used as combo initiator, followed by ult if many targets can be hit, or your q. preferably, use q if ult is up, so you can strike another q when your ult casting is over.
so upgrade your skils depending of situation and your needs

well flash is obvious choice since there is no escape meachanism

second would be depending on your personal skills, clarity if you dont have mana regen runes and you waiste mana without caring much and do not intend going for blue buff at all

teleport if your farming suck and you want to waiste the least xp possible, prevent backdoor, joinning teamfight, uses are multiple

ghost, good escape/chase mechanism

cleanse, great spell on any champ, useless if you face a low cc team, not much use during laning unless you dont plan warding

exasut isnt usefult much on an ap IMO, as you will probably only use it to slow down someone

ignite isnt that useful since fleeing low health enmy can easely be sniped with this champ

fortify/clairvoyance/smite not your role at all, this not a jungling guide

revive/rally NO

pros:

strong earlygame
not item dependant
very high burst damage
range over 9000!!!!!!
can also be close to the action due to possive if build properly
easy to place spells and skillshots, very hard to dodge
low cd stun with lots of damage
aoe ult castable 3 times, can do 1500 AOE damage if used well
nice survivability vs ad

cons:

squeshy vs ap champs
if silenced, you will be completely uselss for the duration
no escape/chase mechanism,
channeling tmie to activate the long range
the speed boost from your w cant be use to chase, the boost is only a compensation for the channeling time

never use your spells to farm or you will be oom very quick. but do it if you got blie buff. be ahead of your creeps if pushing so your q hits all the minion wave

due to low atack damage, you have to practice last hit