Poke with Spiritfire.
Spiritfire when trading.

Wither when getting ganks.
Wither when short trading.

Attack minions when Targon's (the AP one) is off cooldown to sustain, only if applicable.

Get Tear ASAP on first back.

Then get the first Hextech component of Night Harvester.

If ahead or need more damage or wave clear for better tempo, get the tome (middle component of Night Harvester).
If even or need move speed, or otherwise, get Lucidity Boots instead.

Once you have Lucidity Boots and Night Harvester, work your way towards Fimbulwinter which is your next power spike.

The Fimbulwinter allows you to become tankier with the periodic shields and allows you to continue using skills with less worry for mana issues.

During this stage of the game, you want to essentially be harassing with Spiritfire where you can for poke and also be withering for peel or for catch. Spiritfire reduces enemy armor, so spam this as usual but especially be more focussed in teamfights to hit the most targets or the biggest threat. Even a tank Rammus or Malphite with 400-500 armor will be shredded to 250~ armor, they're less tankier than they think.

Most of the time you're sitting in between your engage or bruisers at the front and your ranged carries at the back, staying in range as a threat. You'll be hovering around teamfights or skirmishes and use your HP as bait to buy time for team or cooldowns.

At this stage, you'd be finishing off your last 2 items. I personally go for Frozen Heart if it's applicable to the game, and then a +1 item ranging from anywhere with Randuin's, Abyssal Mask, Force of Nature, Spirit Visage, Stoneplate. Your last item doesn't matter about giving extra mana to synergise with Fimbulwinter, or more Ability Haste for lower cooldowns, it just needs to fit the scenario of the game.

Zhonya's can also work as well as Mikael's.

You do the same playstyle as Mid Game, just catching and chasing targets.