Hi there and welcome to my low elo Nami build!
To start you off, there IS an optimal way to play Nami. Most people I know who succeed at playing her in game have three pretty basic abilities that allow them to maximize her kit.

1) They can land their skill shots. This is hugely important on Nami as her main form of crowd control is an area of effect skill shot. Because of the way that Nami's lands, I would recommend practicing with her before taking her into ranked!
2) How to dodge skill shots. This is crazy important during her early game as she is weak compared to "meta" supports (I.E. Leona and Thresh) and lacks damage output that compares to others (I.E. Annie).
3) Quick cast. I personally HEAVILY suggest you use it, as it allows split second reaction time for engages or escapes.

With that out of the way, welcome to my (fishy) Nami guide!

Runes are pretty straightforward. Gold per 10's can be switched, but since (with this build) Nami is very item reliant it helps you stay on build-focus even if you wind up in a hard lane. Armor reds help you survive a bit longer, ability power blues give your heals a speck more restoration, but can be swapped out for flat MR if you want to cover her low base resistance values. Finally, and most importantly, the scaling mana regen yellows allow you to make up for her high mana costs and low mana pool. It's actually a pretty important part of this build, especially in the late game.

The masteries you use for Nami are a sweet sweet utility tree of massive amounts of gold, and other useful things. Specing 4 into Offense will give you 4% cool down reduction, coupled with your utility tree for a starting 10% CDR. Since your build includes defensive stats, enchanted armor isn't a bad way to go. The rest spec into utility to get the maximum amount of cash that you can, making sure to snag the mana regen along the way.

This is quite a list. I am going to start with each item you should buy in order and explain why. On the offhand chance any of you are reading this part, feel free to send me a message that says spaghetti. :D

First Buy:
Spellthiefs Edge: Because this is a build built for kiting, it should be no surprise that you would get this item. It provides a smidge of AP and some mana regeneration, which are both great for casting spells.
Ward: Just buy the ward, put it in your river bush, and don't get ganked. Let me repeat that WARDS PREVENT YOU FROM GETTING JUMPED ON BY THAT SUPER CRAFTY JUNGLER!
Health Pot: For the heals!
Mana Pot: Unless you are literally determined to waste the passive on your spellthiefs edge you are going to run out of mana.
Warding Totem: Good to ward lane bushes and help prevent a Thresh with the accuracy of a hawk come flying out of the bushes onto your ADC's face.

Lane Phase:
Ruby SightStone: Remember what I said about wards? Well this one lets you place three of them, holds five, and gives you 400 health. The only thing it CAN'T do is tell you why kids love the taste of cinnamon toast crunch (or remind you to use it. Don't worry. Your teammates will)
Mercury's Treads: Great to boost Nami's lacking base magic resist, and also for preventing people from CCing you, they are generally a solid pick for her.
Pink Ward: Great for that one lonely bush in the middle of the river between Baron and Mid that nobody walks in until they're about to do Baron.
Sweeper Lens: Get it at the same time as your ruby sightstone to cast your enemies into the terrifying blackness of doubt, and also make your jungler happy!
Fairy Charms: I may have mentioned a few times that mana regeneration is solid on Nami, and while (wit the runes you have) it is not as much of a problem late game, early game will still not be the best. Buying two fairy charms will let you stay out longer and go harder in fights.

CORE:
Frost Queen's Claim: This item ROCKS. With the 50 AP for healing, 10% CDR for spamming bubbles, and mana regeneration, you pretty much get to go buck-wild. It's active is also super snazzy for slowing enemies in chasing and running situations. Pro tip though, cast it at max range, it is a skill shot and it will go precisely where you click it, not just in the direction.
Iceborn Gauntlet: The only thing better that mana regen is mana! And armor. And AP. And cool down reduction. And a slowing field that will help you more easily hit your bubbles. But most of all it's the look of surprise on the enemies virtual face when you out dual them one on one because they haven't hit tab. (Pro tip. Hitting tab shows you enemy builds)
Rylais Crystal Scepter: Health, AP, and more slows. This item is just an all around solid pick to go with the rest of the soft CC comp being set up with the build. It benefits Nami's stats and gives her more survivability.

Situational/Final:
Twin Shadows: Fun Fact 1: If you type in Spooky Ghosts in the purchase window it will pull up twin shadows. Fun Fact 2: This item makes you move faster which makes it a great scouting item in both offensive and defensive situations. It gives AP. and cool down reduction as well, making it a decently solid choice to round out a build.
Locket of the Iron Solari: An item I like against burst mages, magic comps or defensive help, the active is good for those close call team fights, it's always a solid pick.
Pink Wards: Leaving a spot open is nice, but by the end of the game it can be more helpful to have that extra bit of oomph to your build.
Homeguard Enchantment: Perfectly suitable to get earlier, it helps you get back into fights more quickly, and gives good speed for games played close to turret.
Oracles Lens: Upgrade your trinket! NOTE: THIS ITEM IS A GREAT EARLY GAME BUY IN ADDITION TO PINK WARDS AGAINST A TWITCH OR EVE OR TEEMO OR VAYNE EVEN.

You can start with whatever ability you want, but I find that for lower levels getting and maxing heal first is the most effective. The secondary option is to max your Tidecaller's Blessing (E) but there are very few ADC's who can use it effectively in low rankings until they are put directly into a chase situation. Never max your Q first. The other spells provide more utility and are better with scaling.

For support flash and exhaust are always a must. When running put exhaust on the target with the highest damage per second, when chasing, put it on the target you have a chance of killing.

Ranked play with Nami can be a breeze if you can get into the zen of playing her.

The Nami Creed:
I will not be caught out like a baddie do to lack of vision. I, kite fish, solemnly promise myself to keep bushes warded so I do not become a feeder.
I will punish any ADC that pokes their nose out to hurt my ADC by powerlifting them via bubble to the moon. I will do this until they have to go back.
I will ping my MIAs, even if wards ran out and vision was lost. When wards run out I will creep block.
I will harass but not at the expense of my usefulness as a support. I realize that the best time to harass is to auto attack an enemy when they are about to go for farm, and that this action does not cost mana.
I understand that just because I have a bouncing heal that does damage does not mean I can KS any enemy that gets more than 300 units away from one of my team mates.

Ranked play with Nami is a pretty easy thing to do. The mindset just has to strike a line between buff support and total all in ham craziness. Lull the enemy. Like pufferfish in a shady Japanese sushi bar, Nami is the master of mind games. Act like you are playing a buff support. Hide in bushes, heal and drop your E on your ADC. When the enemy assumes you are a gutless fish, and stick your nose out to harass, punish them for it by dropping a bubble on their head and laying down every little bit of damage you can while keeping your ADC alive. One slip up is all it takes to turn Nami from a bubbly pink heal-based support into an unrepentant killer.

Try to match Nami up with an ADC that does well with someone of her kit. She is versatile enough to work with a lot of lane match ups. I will say, however, if you lane with an Ashe that is even the barest amount competent there should be no reason you lose. As long as you don't feed pre level 6, you are almost destined to get a kill once you time your ults.
In my experience she doesn't do well against Leona early game, as Leona's combo isn't as susceptible to creep block as Blitz or Thresh. That being said, if you can freeze a lane and force a dive out of said Leona you can usually get a free kill for your ADC. She is a great support for teams with Yasuo as she has two abilities that knock up, which makes him able to ult into sticky situations.

Nami's early game is weak. Her power ranges from mid to late game when both her build and her abilities really start to impact team fights. Use this to your advantage and surprise enemies who still consider you just a squish-fish (don't take it too far though). She is great at holding off enemies from taking objectives, or just keeping them around long enough for a team member to pick them off. Nami does NOT do well against champions with consistent dodges, and burst heavy champs. That being said, if Nami survives burst she can almost always escape said champ. LeBlanc is pretty much Nami's worst nightmare, as it is hard to hit skills with such a consistent dash ability. Yasuo is similarly frustrating.

My advice after lane phase is to fight with your team, or, if everyone is roaming, find the most fed member of your team and work with them on getting more fed and objectives. Help someone to backdoor by using your items, wards and passive to set up channels through which you can escape, and also see and impede enemies. Buff your teammates so that they can better dual the enemy, or surprise them with the extra damage output Nami can both give to allies, and also provide herself.

Stop that, you're making a fool of yourself.

*Wait for enemies to cluster, then ult them to get maximum effect. Bubbling a cluster like that will allow your team to output massive damage without sustaining heavy loss.
*Use your Tidecaller's Blessing on allies that have the best chance of chasing.
*Once you have your ice born gauntlet, use it's field to help you hit your bubbles.
*Putting tide caller's blessing on an ADC will allow you to hit your bubbles because of it's slowing effects.
*Always try to bounce your heals ally-enemy-ally to keep any accidental deaths from happening.
*Always remember that if an enemy is between you and a fleeing ally, using Ebb and Flow (w) can speed up your ally and help them escape
*USE YOUR ACTIVE ITEMS OMFG