This is just a quick build for Skarner, which is the one I use. He's an extremely fun, but hard, champion to play.

~Masteries~
-You can honestly build your masteries and summoner spells anyway you want, Skarner will work with any combo of masteries. My favorite is utility because of the extended mana regen, but offensive tends to be stronger and lead to higher scores.

Ghost/Exhaust is my current favorite set up. You can ghost before you use your ult to drag them further, then use exhaust the moment it ends.

Marks:
Armor Penetration; Solid choice, a lot of his damage comes from trinity force procs.
Attack Speed; Always a solid choice for skarner because of his passive and more sheen/trinity procs.

Seals:
Mana Regen/Level; Skarner is extremely mana heavy, if you need mana regen, take it.
Attack Speed; Already explained. Solid choice for any rune.


Glyphs:
Cooldown Reduction; For faster spells, faster trinity proc, all around solid choice, per level or flat.
Attack Speed; Already explained.

Quints:
Movement Speed; He is a chaser, with his shields movement speed buff and ghost, you'll catch anyone, and these runes boost that.
Attack Speed; 10% attack speed, already explained.

For those new with Skarner, here are a few spell combos.

-Using Fracture (E) to mark as many minions as possible, then using your Crystal Slash (Q) to consume all of the marks.

-Use your shield (w) when jungling or farming minions to reduce the cooldown on the shield itself. If you constantly auto attack while the shield is up, you can keep the attack speed boost up constantly.

Ultimate
-Increase your movement speed before using your ult so you can drag them farther. ALWAYS use your shield before ulting for the movement speed increase. If you use ghost, use that as well.

-Use exhaust after you drag your enemies so you they have an even longer time in your allies range.