Hello, this is my first guide so if it sucks don't be a douche.
I play games as Skarner and thought I would share my set-up with you guys to help you out.

I hope it helps!

First off I want to tell you about boot choice which can really help out early game during laning and later in the game as well, you want to choose the boots which suit the type of enemies the enemy team have, for example..

If the enemies have two AD champions in your lane you would want to go for Ninja Tabi also if they are both melee champions Boots of Swiftness or Lucidity Boots will really help out.

But if there are two AP champions getting the Ninja Tabi boots would be a complete waste of money, and therefore getting Mercury Treads would be a much better choice. Mercury Treads should also be picked if you have a lot of enemies who have CC spells as they will reduce the effects of them and potentially save yo' ***!

Other champions in the game should also be considered as you could choose a pair which suit the champions you're laning against but then not be as useful against the rest of their team.

For Summoner Spells I choose Heal and Teleport.

Heal - With heal it seriously helps in the early game to stop a first blood on you or a team mate as you can suprise enemies if they throw a spell and back away expecting the kill and you regain 200+ health, it saves you a lot. Also in the late game you can use it during large team fights to give your team a boost of health when low which could turn a fight around.

Teleport - Having teleport makes moving around the make SO much easier, you can save turrets, get to minion masses before team mates picking up creep kills for much wanted money!

You could also choose Exhaust to reduce an enemies damage output in a fight and/or slow an enemy down, or Ghost, but I think that you get enough Speed Boosts from the 'W' spell and Reverie. So I think Heal and Teleport are the best to pick.

The skill sequence I have put there isn't something you necessarily need to follow, you can pick the spells which you believe to be best in the situation or whatever you like best!

With the items I have chosen here the first items are what I believe are best to use for laning with Skarner as you can use your spells more often to pick up minion kills and damage the enemy champions forcing them to retreat or possibly getting you a kill.
Towards the end of the Item Sequence items such as Thornmail and Force of Nature should be chosen dependent on what damage you are taking, looking at the Death Recap damage taken can really help to decide what you will need - Armor if there if a lot of Physical damage and Magic Resist if Magic Damage is dominant - because if you decide to buy a Thornmail when the enemy team is doing only Magic Damage to you, you will be wasting money and making yourself look dumb. You could also swap around different items with the end game build for more HP or AP, or whatever you think suits the occasion.

I have put the Masteries onto defence as you will be acting as a tank with this Skarner build, getting a lot of CC with items and spells, and the tank Masteries will help to make you survive longer in a battle and continue to slow attacks with Frozen Heart, slow movement speed with spells using Rylai's Crystal Scepter and also churn out the damage with 'Q' and 'E' spells.
But if you would prefer to have the extra AP from Offensive Masteries then you could change a few.

With the runes you are going for survivability building up Armor and Magic Resistance and picking up the Magic Penetration with the Marks as it helps with farming and damage output to enemy champions when using your spells, also because they are the best Rune type for Marks.

When farming in lanes or jungle creeps the item choice of Philosophers Stone/Reverie really helps out as you can continue to use your 'Q' to take the kills and get extra health and kills when using 'W' spell. When in lane you can venture into the jungle to kill creeps if there are no enemy minions in lane for you to kill, but you should never kill creeps if they are to be killed by a Jungler on your team. As it cuts their money and makes them rather annoyed.

When fighting in a team you should aim to initiate the fight and try to split up champions making it easier for your team to take out the individual targets, Skarner's ultimate is a extremely good spell to use when trying to accomplish this pulling away one target from the rest, singling him out for complete destruction. When you initiate a fight you should use the 'W' spell to rush in and draw them towards or pick one of them up with your ultimate.