This will be a bare-bones guide. This guide is designed to provide you with what could be qualified as necessary versus optional and to allow you to adjust accordingly to YOUR preferences. The reasoning behind each item, spell, and mastery is explained. Again, this guide leaves MUCH up to the reader to determine.

The sections below have been organized in terms of priority; what is most vital towards this build and the ideas behind it.

The biggest idea to Support Zilean could be summed up in three words: Endless ability use.

Building effectively with Zilean will go a LONG way to doing well. Many ADC champion players don't prefer Zilean to the other alternatives. An effective bottom lane requires good team cohesion, a similar mindset with your ADC teammate, and that the ADC can trust you as a support Zilean.

Generally, it is significantly better if Zilean can be with an ADC who is capable of holding their own ground. ( Miss Fortune, Caitlyn, or Ezreal as examples) This is because Zilean isn't as durable as a support as someone like Leona or Taric.

By the end, you should end up with a very swift and mobile Zilean who can be at any position at a moment's notice, fully prepared.

Beginning
I choose the starting items above to reach the philosopher's stone sooner, but at higher levels of play, I can understanding choosing alternate starting items. Because of Zilean's Time Warp, Zilean can afford to place his gold toward more urgent matters than movespeed.

Developing
As I approach level 4-7, the items I choose have some variability towards priority and would tier them in this manner-
Tier 1: Philosopher's Stone
Tier 2: Tear of the Goddess (OR Chalice of Harmony), Glacial Shroud
Tier 3: Sightstone, Kage's Lucky Pick, Boots

Tier 1: Getting the gold per second (gps) is the first priority, but simple enough.

Tier 2: This is the big toss up you have to make. If you're buying any big next item, you have to weigh your choices. Glacial Shroud is a GREAT first major item, as all the attributes: mana, armor and cooldown reduction, will certainly be utilized. Doing this though, you are losing out on the mana regen that the Chalice of Harmony or Tear of the Goddess will provide. The con of choosing the Shroud first, IF you decide to get the Tear, is the loss of time toward building your mana pool.

To reiterate, the order of the items you get is preference, but these items are ESSENTIAL to this build.

Tier 3: I have been getting the Sightstone later in the game than I used to; opting for buying wards and placing the extra gold I have at that moment elsewhere. The health that's provided is nice, but not nearly as important as the mana and/or armor that come from the Tier 2 items.

Again, with Zil's Time Warp, I find boots to not be as important as for other support champions. Nonetheless, if you think of buying them before going for the Sightstone, I think either would be suitable choices.

Although the gps can be of use from the Kage's Lucky Pick, I find it almost a waste rather than useful for Zilean. This is merely preference, but recommend trying with and without to see which you prefer.

Final Build
Out of all the items you can choose that suit your style and the game at hand, I would deem these 4 items as ABSOLUTELY NECESSARY for any support Zilean. For an explanation of each item:

Mobility Boots:
This is a MUST for late game, as this serves multiple purposes. (1) By the end, Zilean will be allowed to nearly indefinitely move around the map with Time Warp. Having a player who can hit any part of the battlefield like this is HUGE. Being able to come to a teammate's aid, dealing with a pushed lane, while coming right back to prepare for a team fight, to placing wards rapidly around the field.

Frozen Heart:
With 95 armor, 20% cd reduc, and 400 mana, this item benefits Zilean in nearly every way. You add this mana to the Tear, and the Strength of Spirit mastery points start can pay off decently. I can't imagine any excuse not to build Glacial Shroud for Zilean.

Archangel's Staff:
Now this item could be subsided for Athene's Unholy Grail (or Mikael's Blessing), but one of these IS A MUST. With Zilean's spell kit being decently mana expensive, these items provide the sustainability he needs to effortlessly (and endlessly) go about casting his spells. Choosing the Grail provides Mag Resist and additional cd reduc (which is nice), but you'll be choosing that over the additional AP and mana from the staff (which is more effective with the Strength of Spirit mastery points, if you decided to get them). A third (and definitely valid) option maybe Mikael's Crucible, but I have never used it, so I cannot say how well it works.

Ruby Sightstone:
+360 HP for squishy Zilean with the 3 ward placements he WILL be using as he zips around the map. Can't think of much more to say besides that.

NOTE: At level 15 or below, I try to upgrade my Philosopher's Stone to Eleisa's Miracle.

2 Remaining Spots
With those 4 core items (or even 3 if for some reason you find the Ruby Sightstone unnecessary) being are all that's REQUIRED, Zilean has adequate room for leeway towards how he wants to build the last 2 items.

Does your team need armor? Do you want to further support your carry with Zeke's Herald? Are your AP champions carrying the game? Does the enemy team have too much AD damage? All of these things you can freely consider when getting the game specific items.

Clairvoyance (CV): This summoner spell does not get a lot of love, specifically from many adc's, as they'd rather you have Exhaust. These are the reasons that I personally prefer Clairvoyance

-Zilean has target spells, so he cannot disrupt opposing players in brush without sight (unlike Janna with Whirlwind or Blitzcrank as two examples)
-Zilean has a slow
-CV allows safe placement of wards
-CV helps when your adc teammate chases down an opposing player running into brush
-CV allows your lane to stay safely even when you have run out of wards
-Because Zilean is squishy, CV allows you to safely oppose those heavier support lane players ( Leona, Taric or Tresh as examples). You're squishier, but you range! Make use of it!
- Eleisa's Miracle and Mastermind will have CV reduced to 41 seconds.

NOTE! This summoner spell is NOT essential, but I would personally recommend it for the reasons mentioned above.

This section is meant to cover playing against some specific champions and some methods to fight against them.

Enemy Support Champions
- Sona: Sona has always been a challenge to deal with as an opposing support. Sona's heal remedies Zilean's Bomb. Also, Sona's damage spell does not have to be targetted, which makes trades that much more difficult. Either save up your bomb for the heavy trade and fighting situations, or else you may want to try to place more points into Rewind and/or Haste earlier on than usual. If the enemy team has Sona and you're playing Support Zilean, try to work with your adc in who they may pick. (Example, Zilean with Vayne will just not be able to handle fighting against a team like Sona and Draven)

- Soraka: Soraka has similar nuisances to that of Sona, except with the addition of the silence. Follow the same suggestions, as Sona, except in a fight, ensure that you are on the other end from Soraka to avoid a silence that would prevent you from using your ultimate on your adc.

- Janna: Janna is pretty doable except for her shield. The most effective method opposing her would be to work with your adc to shoot the back creeps immediately after you place a bomb on them. This gives less reaction time for Janna to shield her or her teammate. If this just isn't working out well, stick to basic play; there's no need to risk stealing additional cs from your adc.

- Blitzcrank: If for some reason you're dealing with a Blitzcrank, CV pays off against this "brush-dweller" as you'll be unable to be in the brush yourself. Play safe, specifically until level 6.

- Thresh: Same as Blitz.

Enemy ADC Champions
- Caitlyn: Realize you'll trade one Bomb for one auto attack. Determine if that is a trade worth doing.

- Draven: Either live in the brush (grab pink wards if necessary), or stay slightly behind your adc. Takes no risks against Draven! Specifficaly, if Draven is with Blitzcrank or Tresh, only follow your adc into a fight. They are strong enough together to get you down before you get off your ult.

- Sivir: Should be unnecessary to say, but don't Bomb her.

- Vayne: Consider not staying in the brush. A Bomb and Tumble trade is pretty fair and some health pots should be plenty to staying around in lane.

Key Aspects
- Durability : With Zilean being as squishy as he is, it's worth the extra points here.
- Greed : Necessary for supporting
- Meditation and Expanded Mind : Both help against Zil's high mana cost spells.
- Intelligence and Sorcery: Both very helpful as cooldown reduction is a key component for an effective Zilean. These and Frozen Heart get you to 30% of the 40% cooldown max.

Optional Aspects Many of the points I've placed in my masteries are very optional. The biggest ones being
- Strength of Spirit : If you aim for Archangel's Staff, this is a HIGHLY SUGGESTED. If you opt for Athene's Unholy Grail instead, you can choose to place the points elsewhere (bear in mind what you're losing from this)
- Biscuiteer and Explorer : Depending on your preference, you may rather just choose to place the two points into Wealth instead (+50 gold).

Consider Alternate Points In
- Awareness : To further boost your experience
- Mastermind : With Eleisa's Miracle, CV will be reduced to 41 seconds

Zilean's spells are straight forward in most scenarios. Below are just some basic ideas to consider when playing:

-In the beginning, only use Bomb-Rewind-Bomb combination when a heavy trade/fight is occuring. This takes a considerable amount of mana early on. Use Bomb once and just allow for the full cooldown to take place.

-In the beginning, mana management is vital! Always ensure mana for priority spells. If using a bomb would lower you below the mana required for your ult, or if using rewind puts you below mana for a bomb. Allocating your mana properly will always help keep you prepared for worst-case scenarios.

-In team fights, determine an enemy who you would want to bomb in a team fight, as it may cause them to run away to avoid harming their teammates.

-In team fights, ult effectively. It may pay off to allow your tank die and to save your ult for someone with more damage output.

-Always go for slowing an enemy rather than an ally whenever possible. If not,

-Determine which champion would be best to haste when an enemy is trying to escape ( Nasus would probably a better choice over Caitlyn for example)

The Glyph of Cooldown Reduction is the only main thing I would suggest. Try out your own ideas to see what works for you.