Hey I am Tarranei and have been playing league since season 3.
I main support and have peaked D3 on EUW in s8.

In my guide you will see how I choose to play Nami and why.

I prefer to play Nami and have climbed with her the most, but I do also play Lulu, Morgana, Janna and Soraka if I am against a lane I think that I will lose against, or If nami is banned.

I have mained Leona, Fiddlesticks, Zyra and morgana but climed mostly with ardent supports and therefore have switched.

You will mostly see Nami play on my stream, so feel free to ask me questions about the champion when I am online.

Watch my stream on twitch: www.twitch.tv/tarranei

Nami got a lot cc, which will work with a lot of adc's, this will be an advantage for you if you aren't duo'ing with your adc.
This will also make the lane easier to gank for your jungler, even early in the game.

She got both engage and disengage and is great for both a passive or aggressive laning phase

Nami is therefore also a safe pick, which can be handy with a duo so you can swap in champion select.

Many people like to play aggressive support champions, and since nami is a bit more squishy than normally, you can't be too aggressive against all matchups.

I would recommend to go sorcery/resolve if you play and position yourself in front of your adc to poke against a damage lane and don't like to just be passive and hold lane. this way you will be a bit more tanky against poke, and will get saved. This is also a great choice if you are against champions that can hook or jump onto you, like blitzcrank or thresh.

By choosing resolve you will also get more healing done, which will help a lot against a poking lane.

Exhaust will help your adc not to get oneshot, especially against assasins such as Kha'Zix and Rengar. This will also help you in laning phase early to prevent the enemy escaping, which is handy with Nami's CD's early.

Ignite is also an option, this will be great against heals if you proc it before they do.

Pros: Nami got tons of cc and boosts for your teammates, her W (Ebb and flow)both gives damage and heals at the same time, this is handy in laning phase so make sure to use it.

Cons:She is squishy and therefore you will have to know how to position yourself in teamfights and laning phases. Also her ultimate is slow so you will have to be close or throw it hidden (from a bush or on other side of the wall where enemy don't have vision on you).

Teammates:
Always take a look at your teammates and see if there is any other player with any engage, otherwise you will have to be aware of being the one who engages with your ultimate.

Wards:
use your trinket to ward with early until your support item is able to put down wards, then you swap the trinket out with sweaper to clear wards.
You should also always have a vision ward in your bag, and place them in tribush, at drake before you attack it to clear the area and at baron.

Aqua prison (Q):
Nami Q works great with other stuns so make sure to use it, this could be a combo of caitlyn traps and Q.
Another comco would be your ultimate and Q which will make a long cc for the enemy.

Teamfights:
Since Nami is a squishy champion, you should not be frontline, unless you want to ultimate / engage.
You should always keep an eye on whoever the carry is on the team and prioritize them with your abilities.
Also focus on using Aqua prison(Q) on their carry, if their team is spread out, otherwise try to get as many in a Q as possible.