Xerath is known good as "not best" mid mage due to his immobility. So, what about picking him on solo top then? Well, that position goes for him better due to his passive which gives him a bonus armour making him more resistant on all AD top's just like Darius, Jayce, Renekton or just Sion. At first you start with about 50 armour. Isn't that just pretty much? A lot of mages can go top, but Xerath's passive and mainly W skill helps him on line. How? Just think how good your harrass is, you are attacking with Q from tower and even farming. Small con of it's that he's "mana hungry", but you can everytime ask your jungler for blue mainly when your mid don't just need it.

I wouldn't change runes I have shown you. They are just perfect right now. Armour greatly helps your passive at start of game, and it's better with every minute of game, mainly after combining Zhonya Hourglass. Don't forget about starting 29 AP which helps you to deal moderate damage.

Well, i went 21/9/0 to get more AP and more armour with some health making you more tanky at begin of game. You get also at start 8% magic penetration and spell sword which will help you with farming if you can't just last hit well. Free choice at masteries, you can pick whatever you want.

As we are on solo top we mainly focus on armour. But what about magic resistance? Well, if your enemy on top is for example Rumble or something AP, you are going to change build. How? Start with Mantle + 2 health potions and just swap Zhonya Hourglass with Twin Shadow's or Athene's Grail. Both choice's are good. Don't forget about Rod of Ages and Rylai's Crystal Scepter which both gives you massive Hit Points and some AP. As last 2 items make Rabbadon Deathcap and Void Staff for more magic penetration.
DON'T make Liandry Torment or Sorcerer Shoes due to penetration. Why? You have right now a lot of penetration from Runes, Masteries, your W skill and at last from Void Staff and you just don't need extra penetration. As boots i would do Ionian Boots of Lucidity or Ninja Tabi or Mercury's Treads.

All skill sequence was shown at begin and i won't change it only because other pick sequence is just bad.

As spells I usually take Ignite and Flash.
Spells:

IGNITE - Hell yes, it's good at begin to help you at line and kill low hp enemies. Take it everytime VS champions with good healing.

CLARITY - In fact, it will help you at beginning of game when you mostly need mana, but after 20-25 minute it will be totally useless, because you won't have any mana problem's even with good harrass. I wouldn't take it at all.

GHOST - As Xerath is immobile champion it would be good pick for him. I still prefer Flash, but you will decide what to take.

HEAL - It's right now ok, but barrier would be better.

REVIVE - Nah, it's kind of useless spell but works fine with teleport and prevent opponent from pushing.

SMITE - If you REALLY REALLY REALLY REALLY REALLY CAN'T farm well, that's good choice for you to kill one minion every 70 second. it's about 8 minions in 10 minute. Hell yea!

CLEANSE - Only if your enemy Top has hard CC. Other way, useless.

TELEPORT - That was everytime ok. You can use it to prevent enemies from pushing your lane.

BARRIER - Good choice, mainly vs hard AD like Darius.

EXHAUST - Not good for Xerath, mainly due his stun and range of spells.

CLAIRVOYANCE - Let only support's take it at all...

FLASH - In my opinion, best choice along with Ignite.

Well, first time I thought that solo top Xerath is mainly for fun. But one time I had to play ranked game. On mid went Talon and on Jungle gone Kha'Zix. None AP huh? I could take Vladimir or Cho'Gath but I risked to play as Xerath solo. Blame from team was horrible and Talon couldn't swap with me because of counter issues. How that ended? Well, a penta kill along with good stats and won lane vs really good Nidalee. I think Xerath on solo top is viable in ranked's but he needs to practice before it.

PROS
- Easy harrass on all potential enemies.
- Easy farming with your Q skill.
- Lots of armour early, like late game.
- Tons of damage even without feed.
- Safe line, you dont need to push much due to your W skill.
- You are almost everytime near your tower.
- Good at gank initate.

CONS
- Your immobility can kill you if not buying wards...
- Mana hungry early.
- Small damage amount at first levels.
- Long Cooldown's early

Well, you will not interest much in creeping and jungling because you are not made for it. If you want to take blue or red same you can freely use your ultimate due to short cooldown of it.

Xerath ultimate needs stunned person to get most of it's damage. Your single stun is not enough in big team fight's. You need person with massive CC like Amumu, Malphite, Cassiopeia, Alistar, Lulu etc. With their help your ultimate will be powerful vs all enemy team, but not one target.

What about farming? All you need to do is sometime hit wave of minions with your Q and finish them with auto attacks. Not good at farm yea? Try just to get more kills if you can't farm well, it will fix your money problems.