Main Build

Hexbound Bracers

Hexbound early will let you box with a lot of people. Especially long range heroes that are constantly aggressive. As long as you back off enough to heal, you can get poked all day long. It's also insane late game, as you can frontline for a bit, then take a backseat to heal everyone. I will let myself get hit by an inhib to get mana back so I can keep healing the team as we push the core.

Void Conduit

Void is nice against magic damage, and it will make sure you always have enough in the tank for a thunk.

Truesilver Bracelet

TS is always a must. Getting this item 3rd isn't so bad if you are careful. If fighting steel or riktor, maybe get TS first item. But you will be squishier.

Galaxy Grieves

The boots I just like for the speed and the jumps. I like rotating a lot and I never want to miss out on a team fight.

Crystal Tear

Your main source of healing power, but don't underestimate the Ability Haste as it can help you spam your March more often.

Windcaller

Boy do I love movement speed. It's not the best item for movement speed but it can make a difference and on top of the speed it's more mana regen and more healing power. It gives your teammates a little extra speed to peel them off enemies, which can make a huge difference.

Playstyle

When playing narbash with this setup, I position myself in the front line. I'll typically bait the enemy to attack me or initiate a fight that might not be in their favor. Once I've taken about half damage and successfully body blocked for the team, I will reposition myself to the mid or backline to sit, heal, and peel for my team. When I heal back up I go back to the front and rinse repeat. Doing this gives me unlimited mana, healing, and enough tankiness to not die.

Early Game

Early game is a real struggle for new Narb players. Especially against ranged heroes. You won't have a lot of mana regen in the early game so use your mana wisely. Until you get March, use the Thunk defensively. Once you have march you are going to want to basic attack minions and then use March to get +8 Rhythm stacks BEFORE you Thunk. As this will reduce Thunk's cooldown, and it gives you the move-speed to either engage if you hit the Thunk, or back off if you missed.
Remember you are playing support! Do not engage the enemy unless you and your carry are in a good position and situation to do so. Otherwise you should be Thunking either the enemy and then backing off to pull the minions closer to your tower so your carry can farm more comfortably. If you are facing someone with dangerous CC potential like Riktor, you are going to want to hold your Thunk in response to their actions. Playing this way will make sure that you always have mana for a Thunk and you have a Thunk ready for when it really counts. AND DO NOT LAST HIT MINIONS!

Mid Game

You should have your crest by now. Along with the upgraded wards. You should be warding the jungle and helping your jungler get fangtooth in between minion waves. Make sure you are leeching XP. Also pro tip, you are a human ward! If the minion wave is close to the fog wall and you can leech xp from there, then peek in the jungle and keep a lookout for your carry.

Late Game

At this point you should have enough health, armor, mana regen, and health regen to start rotating around the map. When you return to the battle after backing you need to go to the lane with the closest ally, soak up the xp. Then either stay with that ally or rotate to another ally who is more pushed up. You want to be present for teamfights. If your carry isn't with you, they should go to the lane that is closest to the core to safely farm until they regroup with you.

Situational Picks

These items can be really nice for Narb, but are either situational or require more skill, knowledge, and/or teamwork.

Marshall

If you're going to get this item, make it your first item and make sure you and your carry know what you're doing. With this extra attack speed you can not only push the lane easier, but it's much easier for Narb to reach max Rhythm stacks only using his basic attack.
Here's the strategy: Get this item first, basic attack the minions as fast as you can to get max stacks. As soon as you reach max stacks you burst your march to sprint at the enemy duo, hit them with thunk for reduced cooldown. Since you're at max stacks using your basic attack, you save on mana while maxing out your move speed, thunk cooldown, and healing power. Letting you bully the enemy duo pretty fast. So, you can basic attack to max stacks before healing to save on mana too.
BUT! If you aren't careful, you'll end up making yourself too squishy and end up denying your carry farm since you have to push so hard. And I like to be a couch potato and let my carry farm comfortably.

Dynamo

Just really good. Fantastic even. Similar with Marshall (and even better when paired together) use it if you know what you're doing. And if you are good at landing thunks.
In particular this is pretty good if you are facing a carry with not a lot of escape. Like sparrow. If you know you can absolutely shut down someone and you have teammates that will benefit the dmg. Go for it. And it might be a good idea to pivot to this if they are running anti-heal. Just be bursty instead.

Everbloom

A lot of people run this on Narb. I think most people even swear by it. I think it's nice and all, but I don't think it's a staple. I really only use this if I'm fighting steel or riktor and I can spare the gold. It's a late game item as you'll want it after you have enough mana regen. Good if they have anti-heal too.

Tainted Charm

You're only going to use this against healing melee heroes. Or if you are tanky enough to stick to the carry or supp. I mostly only use it if I feel confident in getting close and they have a lot of CC. Can pair well with a lot of the other options. But mostly build it if you need to tank a Steel, Riktor, Grux, Crunch, Countess, etc.

Tainted Totem

If you find yourself with a lot of mana and up against a team of healers, this item is fantastic. Unless you are building narb to be a front line tank, if so then go Tainted Charm. This item is also really good if you are in comms with a team. Let your team avoid Tainted items in favor of more dmg or whatnot since you'll be supplying blight to the whole team 24/7 by the endgame. Although not that great if your team can't deal damage.

Tainted Guard

Love this on Narb. Especially if you run it alongside Hexbound, Galaxy Grieves, Dynamo, and Elefrost. If you are going up against multiple ADC heroes, or duo carry, this will absolutely shut them down.

Elefrost

This is my backup item for when my carry decides they don't want to participate anymore. Narb doesn't have a lot of laneclear, and while you can go with Fire Blossom to help your team push your carry's abandoned lane, Elefrost comes prepackaged with mana regen, which Narb loves. AND it slows targets! So, if you have a carry that isn't participating, or if you have a carry that is new to the game and doesn't have it figured out yet, definitely pick this item up so you can help push the lane and you can stick to the enemies easier too. I recommend using march, basic attack to slow, get behind them, then thunk, WAIT till the stun drops, then basic attack to slow again, by then you can repeat the process and they won't be moving much. Pair this with Tainted Charm!