Your primary aim is to get your carries fed. Your secondary aim is to become a support carry and do tons of damage.

Early game: You are very weak at this point. Do not let an engage happen until you have a strong upper hand. You must land your spears and aa harass effectively in order to get this upper hand. If you play smart and safe, while landing a good number of spears, your chances will come. Try to give the ADC the kills at this stage. Throughout lane phase, throw traps everywhere... Start of bushes in lane, around the entrances to the lane, in the lane itself. Even though you don't have points in them, enemies will try not to step on them, giving you more zone power. Try and place them where you think the jungler will run through, in places such as the start of bushes or just around corners where he won't see them til he stands on them.

Mid game: Have a wander round. Start expanding your trap matrix and feel free to gank mid or counter jungle with your jungler when you see the opportunity. Nid excels at messy mid game skirmishes, even as a support. Start KSing a few kills if you feel your carries are fed enough to snowball. If they are not fed, let them have a few more kills.

Late game: Once you have a few AP items, start dishing out the hurt. Keep tossing spears and cougar one shotting squishies when you get the chance. Ward and trap everywhere to gain vision, then land enough spears to start forcing the enemy team backwards further toward their base and away from objectives. Eventually your goal is to have wards inside their base and wander around it chucking spears in at them to keep them in there.

Start with your gold item and upgrade it to tier 2 ASAP. You won't be looking to fight early, so start accumulating some gold.

Get Sightstone next for the health and vision. Switch to red trinket at this time.

Sorc boots next for damage and mid game mobility.

Get tier 3 gold item at any time after this... The timing on this purchase is situational.

AP TIME! Deathcap + Void Staff combo for big spears, then Lich Bane for cougar damage. If you reach very late game, potentially sell your gold item for Zhonya's. After your AP items, upgrade your blue stone to red.

Note: As you are support, you don't have much gold to play with and therefore cannot build the same way as midalee. You can skip mana regen items such as Grail and Morello however, because you have clarity. You can get some CDR from tier 3 gold item if you feel you need it. Going hybrid is out of the question... You simply don't have the gold for that. Your build is designed for pure AP damage.

You want runes that will give you maximum power early game, giving you lane control.

Quints: Flat AP for spear damage

Seals: Flat armor for lane trading

Glyphs: Flat AP for spear damage.

Marks: MPen for spear damage, or you could go HybPen if you think you can constantly aa the enemy lane safely.

Full AP masteries give you an strong early and immense late game. Utility masteries would let you reach late game faster among other things, however you would not have the damage needed to really hurt enemies. Some AD in the offense tree gives you stronger lane harass and late 1v1 power. 2 points in block makes your lane phase a bit stronger. 3 points in meditation really helps you sustain through the game, especially early.

Start trap at level 1 in most match ups. Spam them everywhere ASAP. Go spear second, in order to go on the offense and take lane control. Go heal whenever you feel you need it to help your lane sustain... Preferably this is not very often, as ideally you want to be maxing your spears first and pushing the enemies back.

Flash is a must.

Clarity lets you build damage items without first needing mana regen items. This lets your damage remain strong in the mid game, as your gold can be spent on AP. With no mana regen items in the late game either, clarity is required in order to siege. Clarity is regarded as a weak spell on most supports, as it becomes quite useless the later the game goes... This is not the case for Nid, she benefits from it at all stages of the game.