Hey, I go by Empiran or Emperor Piranha on league. I play on oceania server as a Gold 4 player. While my creditability as merely a low gold on a server that is considered to be rather weak, I still believe that my skills and knowledge on Nami may be of help for you.
Currently, I am a lv 7 Nami main with over 220K mastery points on the champion.I hope that this guide will work for you, best of luck in climbing :)

(P.S. This is my first attempt at making a guide, please let me know how I can improve as that would be much appreciated!)

Laning Phase In lane, you want to harass as much as possible, procing your support item whenever possible in safe circumstances to maximise gold income. While being aggressive, it is crucial that you manage your mana appropriately as well as your positioning. Remember to save your wards mostly for the river if you are winning lane (which you mostly will be). At level 3, you can let your adc farm up while you walk up, put your E on yourself, auto, W, auto to proc all 3 stacks of your Spellthief as well as procing your Thunderlord's, giving you a significant advantage and opportunities for Q or Exhaust to catch out low opponents for your carry to follow up and finish.

Roaming If you are a support who loves to roam, Nami is a fantastic pick. The easily applicable slow by auto's with E allows for your Mid or Top laner to follow up with CC to force a flash or just straight out catch your prey. Follow up with Q for securing an easy kill for your team mates. If you are prone to extensive roaming, take Mobi's over Boots of lucidity, and put your E on yourself at all times to speed yourself up, as well as slowing down any threats if you happen to be caught.
After your complete your Unholy Grail, you will be less squishy, less mana constrictive and also heal for a lot more due to your consistent damage.

Team Fighting as Nami Nami excels in teamfighting due to her multiple target healing and CC. You can initiate or peel fights with your Q or R, as well as supplying your ADC with empowered auto's. You want to position yourself behind your team with this aggressive build as you will be rather squishy, keeping up your healing and supply of damage. In a teamfight, hold on to your Q until the enemy ADC or main damage dealer stands relatively still to cast a spell or auto attack, as this way you will make the most out of your CC. If your team is wining the fight, W onto an enemy first for increased damage; if your team is losing, you should tend to W more to allies for increased healing.

Thank you so much for taking the time to read my first attempt at a guide. Hope this was of assistance and I wish you the best of luck for climbing!

Empiran out