This is my first attempt of doing a quick guide how I play Jarvan IV, The Exemplar of DEMACIA.
Jarvan in my opinion is just a bully, not a damage dealer, so I prefer build 1, but many would prefer build 2.

I got no real runes for Jarvan, I use my AD set, with some magic resist and mana reg, just for the early game harassment.
My masteries follow a standard 0-21-9 build, for the sake of tanking, as you will CC heavily and you will get some damage focus.

My itempath in this guide is very overall, this is how my standardbuild looks. But very rare occations will occur when it's actually useful, the enemy team deicides your items.
I usualy face 2 AD, 2 AP, 1 tank, therefore of this build is this way.
If you face 3+ ADs I'd recomend changing Randuins Omen into a Thornmail, and ignore Armor after that, if you face 3+ APs you should change Ninja Tabi into Mercury Threads, and Eleisa's Miracle into Quicksilver Sash, Spirit Visage, or Wit's End, depending on your, their team.

I max R>E>Q>W
Demacian Standard for harassment, Dragon Strike to throw then into the air, Golden Aegis to slow them, and block some damage, then just auto attack till you feel comfortable. Use your ult when you feel it's time for a fight within the Arena, onto life and death.

Jarvan has great survival, therefore I use Ghost Flash, so I can rush into a full enemy team, CC them, survive, and either run or keep being annoying, very useful to risk your own life to save a more valueable hero, then run away.

I always go mid, yes with Jarvan.
Only last hit & harass til lvl 8, then you can start pushing.
Attack each ranged creep once, for your passive, use Demacian Standard, and finish them with Dragon Strike.
And simply kill the melee ones.
THIS IS VERY DEPENDED ON WHAT YOU FACE, AND YOUR ENEMIES AGGRESION LEVEL!
Always priorize Demacian standard on enemy Champions>Creeps.

My way of playing Jarvan IV is simply as a Tank, CC, and bully.
Rush in, get as many as possible knocked up, slow them, hit everyone once, and eventually ulti as many as possible inside.
This doesn't deal much damage, and you will take alot, thats why I build him tank.

If you have any questions, suggestions, feedback feel free to comment.
This is also a custom made way of playing Jarvan, many people ingame have complained on me for doing this, but I rarly have a bad game, even to I rarly get kills either.
Sadly you need your team, BADLY with this build. Your job is to make them unable to kill your team, not kill them.