Hi, I'm Eden, and if you've came to this guide, you don't need all the creative flair and imagery and everything else that almost all guides come with.
You want something straight and to the point. Easy to follow and understand.

I hear you, fellas, I hear you.
Item builds, rune ideas, and brief explanations as to why stuff works the way it does. That's all you need.


(This guide assumes that you know basically how to play the champion, and you need a little bit of an idea for some extra 'oomph'. If not, I'd recommend Jovi's Bubble Pop Nami guide. Lots of explanations and pictures. Even videos. What fun.)

Staples:
Flash is key on nearly everybody. If you're not using it, you're beyond whatever help I can give.

Exhaust is my other spell of choice on Nami. Used properly, it can allow for setups for your Q, disengages from problematic enemy champions, and negation of burst damage. I take this spell most all of the time.

Substitutions:
The only one worth subbing in is Ignite. Against a particularly squishy or healing-centric lane, you can use it to add in a significant amount of burst damage and deny healing/lifesteal. Sub this in against Soraka/Vlad/etc.

Sorcery:

Summon Aery is essentially a weaker, point-and-click version of Arcane Comet that does less damage, and also has a shielding component when you cast one of your spells on an ally. This not only grants a heal/shield combo on your W, but also allows your E to shield people. It synergizes well with Ardent Censer by allowing two of your abilities the potential to increase attack speed.

There isn't much to say about Manaflow Band, really. It gives you max mana when you hit enemies with abilities, stacking 10 times, and gives you a small mana regen passive when you hit 10 stacks. Helpful because Nami burns through mana like crazy.

I tend to use Absolute Focus on Nami more than any other because I usually have my own HP topped off enough to use the extra AP. If you don't, you might get more use out of Celerity, with the added MS and MS to AP ratio it gives.

Gathering Storm, because Scorch only applies to poking, where Gathering Storm buffs all of your skills. No contest.

Inspiration:

Biscuit Delivery is excellent for heavy mana-use supports like Nami, since it gives back 15% missing health and mana on use. Keep in mind, the amount of HP/Mana you restore is determined as soon as you use it. If you are healed (by a normal potion, Heal, or something) or have your mana restored in another way (natural regen, some goon using Clarity), you will still regen the same amount based on what HP/Mana you were at when you used it.

Magical Footwear is very helpful for its extra movement speed bonus. Take this if you think you're able to get some assists in lane, otherwise take Time Warp Tonic.

If you've made it this far, you know I'm not explaining trivial things like what her abilities do.

Maxing Order:

Take your W or Q at level 1, depending on the lane you're in or if the jungler invades and nobody knows what's going on. It be like that sometimes.
Take the other one (Q or W) at level 2, then take your E at 3.

Max W for healing and damage boost, then E for the slow bonus and extra magic damage, then Q, since leveling Q only improves cooldown and damage, not stun time. Take points in your R at 6, 11, and 16, unless you're insane.

Ardent Censer is the staple item you'll be running in this build. Providing attack speed to your ADC (or literally anyone else) is always a bonus, and you'll want to keep that 6 second bonus stuck to them as long as possible by alternating W and E casts with Aery.

I consider Boots of Mobility to be essential to Nami, due to her ability to set up picks for her team with Q, E, and R. You gotta go fast to capitalize and get from point A to point B, right? Right.
Also helpful for setting up wards, then bugging out without getting blown up.

I personally love Twin Shadows, since it not only slows (up to) two people, it also reveals them when hit. Helpful to assist in sticking to a target, and for objective control/scouting. The obvious downside is that the little buggers that come out can occasionally just hit one person. Overall, it's a playstyle choice. Sub out for Shurelya's Reverie if you want.

Frozen Heart is a greatly needed bunch o' armor with a built in attack speed slow for all those pesky Kai'sa's and Vayne's you see all over the place. The 100 armor is also helpful in surviving early damage from lethality builds, but you might want to sub in a health item or two to make the armor worth it.

Locket of the Iron Solari, occasionally known as the "Give this teamfight a chance" item, is exceedingly useful against AOE team comps, and even more so explicitly against champions like Karthus or , who have an entirely AOE burst-heavy kit.

Hopefully this helps you out, if you came here for what I hope you came here for.
Vote up if you like, or get back to your game if you don't.

Look forward to more guides like this when I have time to do them.