Pretty simple, this is a poke/defend build with emphasis on defend. So stay safe and keep your carry safe. Your not super fast yet so play passively and look for openings.
If they get to agro against you just set a Q aimed down lane and let it charge. They will keep their distance till it goes off.
Charging one in a bush can give even more buffer room.
Watch the other team carefully for any tells of impending skill shots or incoming poke and shield your carry. This is one of the hardest parts but can negate a huge amount of damage and allow your carry to trade.

This is where you play the Vision game. You will buy a good number of Vision wards for the rest of the game so make sure you have one up at all times. You will also be roaming a lot so watch the mini map for opportunities to be the most useful. This can be collapsing on a fight or helping a chase/escape. You should have decent speed by now so try to position yourself to the best advantage. You want to be able to speed your allies up with your passive but that only works if they move towards you. That doesn't mean you have to be right in front of them though, even if your off to the side as long as they are moving closer and not away from you they will get the passive. Try to make sure you get near minion waves too, gotta get that passive gold and EXP.

By now you should have your Talisman and your Twin shadows built make use of them as much as possible. The cooldowns are fairly short and they are invaluable for both chase and escape.

You will have even more speed by now as well as more CDR and AP. This is where your shields should be especially powerful thanks to Ardent Censor. Remember also that by this point you are super SQUISHY and more than ever you need to pay attention to your positioning as you can be one shotted by some carries, and it would only take 3-4 hits from bruisers or even tanks to take you out. You are a back line support at this point. Pay close attention in fights as some enemies knowing they are as good as dead will throw themselves at you with all they have to get something out of the trade. Make use of this fact if your brave and can react fast enough. Otherwise, More wards, more roaming, more map control, help where you can even if it's holding back a minion wave till another champ gets there. Last hit if you can but try not to use too much to wave clear unless you must.

Luden's will be your last item to finish, don't even start it till you have the oracle lens and I would recommend waiting till you have Ruby sightstone. But let me stress this. That last spot should be mostly reserved for vision wards. They are basically your last item until super late game, after everyone is full build, and you can just buy a Luden's outright.

Pros: Fast, Lots of utility, Can always do something, Cheap build as it only costs about 12k gold to finish, you will get tons of "Teamwork" points.

Cons: You are very team reliant, if they fail you fail, if they are compatent you can enable them to be godlike. You are the definition of Squishy, You have almost no damage past level 6 unless you shield yourself and even then that becomes moot by 10. Your abilities are utility only till you finish Luden's at which point you will have some damage but nothing to write home about..No pentas for you, get used to it.

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