Hello! I'm not a great player of LoL, but I've looked into this build extensively and play it often. It usually gets some wins, though it's very much an off-meta build and an aggressive one at that. You and your ADC won't have nearly as much sustain, since unlike other supports, you don't have a shield (like Morgana, Yuumi, and Sona), or a hook (like Blitzcrank, Thresh, or Nautilus), or a heal (like Yuumi, Soraka, and Lulu), or an instant CC (like Morgana, Lulu, and Sona). But what you make up for by not having any of this, is that by correctly building your items, you're able to output an ungodly amount of damage, and you're a better lane bully than most other supports. The downside also though, is that to have this much damage, you're going to be very fragile and weak. Thus, you're a glass cannon, or rather a Glass Kennen. Sure, you output tons of damage, and in a teamfight, you'll help your team out with a bunch of CC and damage, but if you get caught out and fail to get away then you're dead.

Since we're trying to essentially maximize damage, the only good support spells you want are ignite and flash. You'll want flash to be able to flash into the enemy team with either your E or your Ult. Since both are starters for getting the enemy team stunned. And you'll want ignite since that'll provide damage that might get a kill, or dissuades enemies from coming closer. If you really don't trust your laner then you can take heal instead of ignite since it'll allow you to recover from misplays sometimes.

Your ability order is going to be Q<W<E. You want to max Q first since it's good at lane bullying and zoning off the enemy team. Then you want W since if anything is currently marked by your passive, then they'll get another mark, and they'll take a lot of damage. We don't max this first because this also counts if lane minions are marked, and we don't want to steal CS from out ADC. We then max E, since it'll allow you to get some speed towards enemies, or even have some escape possibilities. You'll also find that using E can help you when using your Oracle's lens to hunt for enemy wards.

You could decide to max E second and max W third, the problem being that E is a very all in ability, and thus is a good late game ability since there are more teamfights where you'll need to go all in.

I find there are three categories of Glass Kennen, there is the normal Glass Kennen which mostly builds AP/Magic Pen Items whilst having some Sustain through Zhonya's and Banshee's. The second category is full Magic Pen, which has all the magic pen items, as well as items that burn your target and apply on hit effects, where the only defensive item you have is your zhonya's so you can protect yourself a little. And the Final category is on full on hit effects. This is where you second item Guinsoo's and then buy items that provide AP as well as on hit effects, where you'll be basic attacking for a ton of damage or even burns. Your enemy team will hate you for it but you'll be the most fragile of the 3 types.

Whilst the whole idea of this build is that you're heavy output damage, I don't think you'll find Dark Harvest to be a great choice for this build, at least not as good as electrocute. You'd normally want Dark Harvest if you're going to scale massively, and since we don't have anything that continually scales, we'd not get as much use out of it as we do from Electrocute. Because Electrocute does bonus damage every 3 abilities or attacks, which is the same amount needed to proc our passive's stun. Meaning they'll not only be stunned but also lost a good chunk of health, leaving your laner or anyone else even to come in and finish the job.