Fast explication to become an OP skarner:

1.You most be the tank during the first battle be using your elixir, giving 140 hp, and your hp potion, 200 extra hp during the gank (use it just after the battle start, it's more useful)
2.Spaming your first skill is good against champion, but a waste of mana against minion in early-game
3.Your shield is useless during a battle, use it only if you want run, or attack faster against an turret. When you reach this skill lvl 3 or more, use it in battle, it's can be usefull for block some attack and refresh your other spell (attack speed boost)
4.The third skill, Fracture, can be used for heal your-self after an gank, during an battle, or just like an AoE for get some gold with minion (do it only in mid-game, if not, you will run out of mana for fight).
Tip: Crystal slash can consume the mark for heal you faster without lose your time to hit each minion
5.Your ult, an really useful attack, is the perfect skil for start an battle. Grab your ennemis and run near an turret, the turret will attack him, and you can slow him with Crystal slash, or if you can, tell to your allies to stun him, and kill it. You can use it to chase ennemis and give him to your team for finish it and get an kill.

6.This build is not the strongest one for noob, for people who prefer an powerfull DPS, for the people prefer ranger or caster. Don't forget, skarner is not immortal, but very strong. Don't go alone anywhere or try something solo, you are an champion like an other...
but at least he is OP