Skarner is an interesting JG as of season 16. He can evade vision with E and has great gank potential. His R allows him to position hit enemies as you need.

Pros: Great tank (or bruiser), Great clear speed, Can move enemies, Synergises with a lot of champions, Very strong ganks
Cons: Shut down by antitanks, Vulnerable to fed botlaners, Hard time soloing objectives, Cannot carry on its own

Q: Pick up a rock, next three autos are enchaced with Tiamat effect, third one slams the rock dealing 9% max health phisical dmg and applies slow. Can be recast to throw the rock, dealing dmg in area and slowing down.

This is your main damage source. Do not underestimate skarner's damage potential early on.
Time limit for the AA resets whenever you E.

W: Gives you a shield and deal damage in an area
Just cast it on cooldown during a fight, ok?

E: You go forward, ignoring walls for a set amount of time. If you hit a champion or monster: push them along. If you push them into a wall, they get stunned

First, do not use this immediately once you have enemy in range. Istead, get as close as you can then E. You have easier time hitting them and also can drag them furtrer in.
Second, slam into a wall. The cooldown is long early on, but slamming into a wall with enemy shortens it considerably.

R: Charge up then skewer up to three champions in front of you for 1.5 sec.
You can move with them and you gain extra movespeed

Use this only if you know you wil 100% hit the enemy. It is one of the strongest CC tools in the game.
When you are in a smite contest over an objective, you can skewer enemy JG just before the time for smite. They cannot smite during supress.
Also please, try to not move too much during R. Sometimes it is best to just hold them in place

Keystone: Grasp
Helps you during fights, as you AA a lot
Keystone: Aftershock
If you slam into a wall, you get it of for free

Shield bash synergises with W
Conditioning for better scaling, can be swapped with bone plating or second wind into harder matchups
Overgrowth is the only useable rune in the last row

Secondary runes: Inspiration
Boots for free are a must. You get free 300 G + 10MS on top.

Second is either approach velocity or tripple tonic, depending on how fast you need to be
For faster enemies, approach velocity makes your life easier
Into more immobile teams, you go tonic. Free gold, extra power for one fight and a free skillpoint is a lot of value.

Tank: You start heartsteel. Then you go boots followed by Jack-Sho and burn item.
After that, you have two slots to fill up with resitance based items.
Damage: Triforce into boots, then Jack-Sho followed by Titanic. Final two slots can be filled either with bruiser or tank items, based on if your teams has damage or not.
Triforce gives you great synergy with Q. Best when you use Q, two AA then finish with a throw and a third AA.

First clear: Start on the opposite side than enemy JG. If you have no idea where he starts, default to blue.
Do the standard clear route. If you want, you can go crab between redbuff and crugs or blue buff and gromp. (deoends on where you started)

All game, avoid 1v1 fights as much as possible. You do not have damage on your own. Even if you build dmg
Gank anytime you have the opportunity, your best chance is to feed your laners, and let them snowball.
Gank example: You finished crab, and see that enemy mid is pushing toward your mid tower. Your mid is clearing the wave under tower. You go gank mid, and E the escaping enemy midlaner into the turret. Your mid gets easy kill from it.

Try to keep vision in pits. Your R gives you an edge when it comes to stealing objectives. Knowing enemy is on dragon gives is a must for this.
You can solo first dragon, grubs and herald at spawn time.
Past herald, stay with your team. You excel in teamfights.

In teamfights, you goal is to isolate priority targets, so your team can make quitck work of them.
Ex: Enemy has fed Caitlin. You have fed Yone. You E Caitlin into a wall, then R her and drag her to your yone, which removes her from existence.
Use R on as many enemies as possible and position them into your team's abilities. (Or drag them under turrets :)

You are not the carry, you play for the team, and team in turn plays for you.
You do not fight 1v1.
You try to not move to much during R.

You struggle into any tank shredder (Viego, Rhaast, Warwick...)
Vayne shuts you down completly (Her E stops yours and she just melts you)
Other matchups are team dependent

You synergise well with champions like MF (You hold enemies in her R)
Notable synergy is Veigar, as you have more than one vay of getting enemies caged by him.

Skarner is a nice pick for when your team needs a tank. Or CC.
He is easy to pick up. Try him out and see if you can drag someone into your turrets (Bonus points if you take them into fountain)