This is my first guide and I am doing it just to test the site. But yeah, this is the build I use for Nami and I noticed that my Nami heals way more than other Nami's around. So if you want to know how this build can give you insane healings, just read below:

The great masterpiece in this build is the new Athene's Unholy Grail. It's simple: when you cause damage to an enemy champion, the item stacks. Your next heal or shield on an ally champion (except yourself) will consume the stacks to heal the ally. Also, Athene's Unholy Grail gives you Ability Power for each 25% Mana Regeneration. This Ability Power itself will power up your heal a bit, but most importantly, it will help you get stacks even faster.

Athene's Unholy Grail gives you some mana regen, and thereby ability power, but buying more items with mana regeneration will help you get even more ability power. The second great item is Ardent Ceaser. It powers up your heal by 10% and your heal will give allies some lifesteal and attack speed (more healing). Ardent Ceaser gives you ability power AND mana regen so I can tell at this point your AP and ManaRegen will be kinda high as well as your healing.

Buying Sightstone is essential for almost all support champions, and with Nami, buying the Eye of the Watchers will give you the wards and mana regeneration, wich will also give you AP due to Unholy Grail, and again powering up your heal a bit.

Redemption here is also powerful.It itself powers up your heal by 10% (yeah it will stack with Ardent Ceaser, their passive names are different), but it also gives you mana regen, which makes Grail give you AP and again your heal becomes stronger. Redemption's active is also very useful.

Now about the boots: Eye of the Watchers, Ardent Ceaser, Unholy Grail and Redemption give you 10% cooldown reduction each. So with this build you are already with 40% and Ionian Boots will be useful only with the Summoner Spells cooldown reduction, that's why I rather buy Boots of Mobility for getting faster to help my allies.

With these 5 items you will have lots of Mana regen, AP and heal increasing, and for the last item you must go for something situational, but if you don't know what to buy, I think Locket of Iron Solari is great for almost every situation, cause you will be more resistent and you can shield your allies in a team fight.

I know that some people like building Frost Queen's Claim and Ruby Sightstone instead of Eye of the Watchers, and I think that's totally OK. Frost Queen's Claim will give you some extra AP and it will be helpful, and you will still have wards thanks to Ruby Sightstone. But I personally like Eye of the Watchers so I can buy something situational (e.g. Solari).

Unholy Grail needs some stacks before it can power up your heal. You have to deal some damage to enemy champions before healing your ally.

1) In lane phase you can use your W on an enemy when your adc is not at boucing range. The damage you stack Grail and healing yourself will not consume the stacks. If you do want to heal your adc so do it when he/she is at boucing range, but be sure your adc is closer to the enemy than you. It can be a good way to poke, but remember: your W has a high cost of mana, so don't spam it until you have the first items.

2) In a team fight, you can start with your R and right after Q on a carrier, so it will be stunned for a longer time. All the damage dealt by R + Q should grant you the 250 stacks, and you can W on a hurt ally and see the magic happen.

Your E will NOT stack Grail, since the damage is being dealt by your ally, not you.