You play this like your the ADC. Let your ADC know about the strat in champ select or if your with a friend it's even better. Early game you're pretty useless. Let your adc champ poke and lvl 3 you can go in for some trades. Important thing to note is to make sure you're always q'ing them and get glacial shroud as your first back item every time (builds into frozen heart). Lvl 6 if the enemy is grouped together its better to all in them then but if your picking off one there's a good chance you'll get both of them. I personally run this with a Varus and the airborne into the root is sick. If you have banshee's for the extra AP in this tank build you'll kill one (or 2) instantly. Also, you can 2v3 ganks if both ult's are up.

The second your ult hits you want to e then q to slow one. Then follow up with an ignite (if they're not already dead from that) and w to pummel them to death. Your adc will be at a safe distance doing good damage.
P.S. This works best with a friend so you're able to communicate good and tell them who to run.

When you roam you want to have you're R up since that's your main source of damage. If your roaming mid to a melee with little cc then it's a guaranteed kill every time. Make sure you're mid laner knows exactly what your doing (ping ult and who the enemy midlaner is and signal you're coming).

In team fights you're the person who engages. If all of them are clumped up like in low elo's and you hit you're ult on most of them you win the fight. That's pretty much it.