To use this build, you should already know how to play Renata Glasc.
This is not a guide on how to play Renata Glasc. It's a guide on how to use Renata Glasc as a Bot Lane Carry.

Only follow this guide if already you know how to play Renata Glasc.

I know what you're thinking already. "This delusional idiot on the internet wants you to play AP Renata and troll your team."
And you are.... 1/5th right with that statement. The only trolling you'll do is that you will be trolling the enemy team when you kill their tanks like a Kai'sa while making their actual Kai'sa kill her own support.

The goal of this build is to exploit the AP scaling on your Leverage Passive to become a max health damage dealing marksman-enchanter hybrid who can carry teamfights through utility AND damage. And you would be surprised how effective you can be with that. I mean, just look at your numbers!

This build is best played in the Bot Lane as a carry because you simply have access to more gold. You will be able to reach power spikes faster and enable your team to do huge damage that way.
As a support, you will simply be weaker at every point of the game, meaning you may as well build traditional support items.
Furthermore, your playsyle is going to be the same as any other marksman, because your passive, which applies on auto-attacks, is your main damage.
Your other abilities are important, but you functionally become an ad carry, so playing in the marskman-dominated role is just the best fit here.

Your Passive AP Ratios are comparable to AP Varus, which becomes apparent when you hit full build. In teamfights, you can melt tanks as if they weren't even there, which makes you an excellent front-to-back teamfighter.
In addition to bringing a lot of damage to teamfights, you also have the utility of a support, being able to shield your teammates and give them a second chance through your W. Your Q should be immediately recasted unless absolutely necessary, because hitting the Q will lock you out of auto-attacking, which is not something you should be aiming for as APC.

Lastly, as Renata Bot, you have a lot of synergies with other supports that traditional Marksmen and even Mages don't have.
You are able to stack your shields on top of enchanters, empower certain ally abilities, out-sustain your opponents, follow up on ally hooks with your own, the list goes on.
Your ultimate is slow, but if you couple it with another ultimate like Seraphines or Namis, your R becomes utterly unavoidable.

This playstyle has its own unique challenges, but most of its issues slowly disappeared over the last couple of changes, actually. Let me detail it this way:

Firstly, your waveclear is pretty bad before your first back. Until you rush a Pickaxe, you don't deal enough damage to minions to reliably farm under tower, and your E has a high cooldown, so you need to be mindful when you use it.

Another downside is that you are chained to at least one teammate. This pick is not supposed to fight by yourself as you can not proc your passive on your own. As a consequence, 1v1 fights are next to impossible.

Lastly, you are both immobile and squishy. You can fix the latter by building defensive items, but until you finish your core build, you will have these weaknesses and just need to live with them.

Over the course of the game, you'll encouner a few peculiarities of this playstyle.

• Despite the weird picks and the weird items, your playstyle is the same as that of a traditional marksman. Your support may get funny ideas of what kind of things you need, but they are wrong. They just simply need to play as if they played with a normal adc, and it'll work out just fine.
• Unless you are way behind in farm, you will want to stick to your team like a piece of gum. You are practically useless in 1v1s, so only split off from your team if there is easy gold to be farmed, then immediately join them again.
• If you have any allies with built in Damage Over Time, these will be your highest priority. DoT instantly procs your passive. Additionally, Ivern, Leona, Lulu, Milio, Nami, Sona and Yuumi can instantly proc your passive, as some of their abilities apply from your Auto-Attacks.
  IMPORTANT NOTE: Renata's Passive can sometimes be procced by ally item effects, but this changes from patch to patch, so it's extremely hard to tell if building a strategy around this is reliable.

If Renata gets nerfed, I'm driving all the way to Riot HQ to play the most annoying sound in the world at full volume the whole day. That being said,

Good Luck and Have Fun