Hi guys,, I have played Nami since I started playing league and I am not sure if this can be called as a guide as I am not a pro-player but I love Nami XD. I will try to let you guys know what I think is the best build for her from my experiment.

After maxing W, max E if you can't land your bubble(Q) in the game.

Combo:

(E)>Q>W>R >> If you can land your bubble(Q), it will stun target(s) for 1.5s. If you dont auto attack or use your W and use your ult right after your Q, the CC effect will become shorter than it could be.
P.S. use E can slow enemy or can speed yourself to make it easier for you to land your bubble(Q).

R>Q>W>E >> If you cast your ult in team fight, use this set of combo due to the range of Nami ult is really large and the slow effect can really make a different for you to land your bubble(Q). Btw try bubble the champion that you think will do the most damg to your team and exhaust him (often midlander/adc). Dont use W to hit enemy if your teammate(s) is/are in low health. Try safe you teammate than do damage due to you are just support you can't really do more damage than your teammates.

I know many Nami player often use 0/18/12 but I like to play Nami as a poke support not just a healer. Moreover, you can heal yourself with 30% of AP. Therefore, if you know you are not gonna one shoted by enemy champion,this masteries should be fine.
P.S. tbh I don't think taking 0/18/12 can avoid you get one shoted.