This is my first guide i hope its not too bad :)

Mastery page
The reason i put 21 in Defense is for the final perk Juggernaut and have enough points left over to give me 9 to get 10% magic pen for Skarner's abilities making him a little more balanced between taking damage and harassing. i don't take attack speed and armor pen because the bonus magic damage from crystal slash does more magic than physical. also i personally see people getting armor items to try to counter me which is why i sneak AP in there to counter them countering me.

Rune Page
I focused mostly on his physical attack's with magic resistance to help make the defense ratings about equal and attack speed to help lower cooldown's in those brawl's. and the movement speed to help keep them in the AOE range to slow them.
!*!*!*!*!UPDATE!*!*!*!*!*!
I decided to try a little more defensive rune page and it seemed like i was surviving more by running from the failing team fights trying to not give them a kill

Item's
i always start with Regrowth Pendant and a potion.(some times a mana potion if i have a good lane partner)then as soon as i have enough gold for the philosopher's stone i recall and buy it along with maybe another potion based on how I'm doing and who I'm fighting. the boots he help me keep up with fleeing enemies, and then Sheen to do chunks of damage while im spamming Shattered Earth. I will finish off the boots(i generally go with Mercury's Treads unless its a heavy melee team then ill buy Ninja Tabi and later buy Eleisa's Miracle for Tenacity). then i start buying the tank gear Warmog's Armor and starting pieces for Frozen Heart and Force of Nature. by the time I've purchased the 2 starting pieces of the 2 armors ive become a real annoyance.
!*!*!*!*!UPDATE!*!*!*!*!
I worked a build in for using the new item Athene's Unholy Grail that's under the 2nd Skarner. It really makes him a lot more offensive and solves any mana issues that you or i ever had with Skarner

skill's
i start with Shattered Earth because its the staple in this play style. making sure to keep the buff on as much as possible. if I'm chasing some one i will hit a minion with Shattered Earth as i run by one. as for Seismic Bastion i max 2nd b/c movement speed and attack speed is the greatest thing ever for Skarner.(the damage soak is also a plus :P) to be perfectly honest IMO Ixtal's Impact is garbage it cost to much mana and doesn't really do all that much damage until its maxed and even then ill only use it if runners are getting away with low health. and remember the faster you move the farther you can drag a champion with Impale

Summoner spells
i take Surge A MUST HAVE and Ignite to help with early runners and champs like WW or any other heavy life gain champs. or if your new to Skarner take Heal till you learn him a little better. take Summoner's Wrath to make Surge and Ignite outstanding with a boost of AP and AD to make Shattered Earth even more powerful, and at lvl 1 with Shattered Earth if you pop both Surge and Ignite on a single target your unbeatable as long as they don't have Cleanse. i will also use Surge to help bust down a turret because breaking a turret down is more important than saving it for a possible champion kill.
!*!*!*!*!UPDATE!*!*!*!*!
i got rid of Ignite for Flash to solve the problem with champs getting out of range of my Shattered Earth and with Athene's Unholy Grail i dont need the boost you get from Ignite with Summoner's Wrath

Jungling
Personally I'm not the best jungler. so around level 3 ill start picking off camps in between lane pushes. If i have Sheen at level 5 i will take blue buff and start noming faces. if i think i have enough time ill grab red at level 6. If i have blue buff at level 10 ill take on dragon solo with a health pot after my first Seismic Bastion wears off. or if i have both blue and red i wont need the potion for that extra health regen.

Twisted treeline
When i do Twisted treeline i do every thing the same as if it was a classic match tho around level 3 i start jungling in between lane pushes. this allows me to keep up or get a head in levels and charging out of the jungle(generally top because it allows me to keep an eye on they're jungling and green buff is hands down a game changer for skarner. around level 5 ill make an attempt at killing green. with a green elixir. i will also try to convince my 2 other teammates to help kill dragon around 6 or 7.

Dominion
If i take Solo bottom ill buy Sheen right off the rip and use Seismic Bastion to get to the first nod as fast as the other guy who bought boots.(unless they fully upgraded the boots then its a little behind.) if i go with the team ill buy the blue mana crystal the first pair of boots and health crystal saving up for sheen with the remaining cash. after i get Sheen and a pair of good boots ill start building Frozen Mallet then the Sunfire Aegis. then Force of Nature and the last item would be Frozen Heart.

Late game
Some times in those late games you might consider selling Sheen instead of upgrading it to Trinity Force for the Hextech Gunblade. it will help you stay in those team fights and you gain quite a bit of health from Shattered Earth. i personally like Trinity Force but i have had some interesting turnarounds when purchasing the gunblade. also its activated ability allows you to damage a champion if you grabbed them with Impale to help solidify those kills and slow down runners if they are just out of range of Shattered Earth since the gunblade as a slightly larger range than Shattered Earth.

I hope its been informative and maybe helpful to you in some way :) please rate this and comment as well id like to see what people have to say maybe some changes you'd make id like to read your thought's :)