Hi Guys sorry if it seems a bit off but this is my first guide thought i'd make one only because the other guides i have seen don't have the Rod of Ages included in their guides and rod of ages is really good for Nunu. so here we go. Please note if you didn't already know this is only a guide you may change to what you think is best.

always go Doran's Ring first because the extra ability power, Mana regen and health for the early game push.
for the second item i go for Sorcerer's Shoes for the spell penetration but you may change depending on the team you go against.
The next item you should go for is Rod of Ages for the extra ability power and the Health and Mana are good for Nunu.
after rod of ages you can choose to use banshee's vale or Void Staff] depending on the team if there is a lot of Magic Resist then i would go with the [[void staff before banshee's vale. otherwise the banshee's vale is good to stop other champions from using a stun or silence effect against Nunu while using your ultimate ability Absolute Zero.
Now to start off the Rabadon's Deathcap with a Needlessly Large Rod once purchased this you may finish the rabadon's off.
Last but definitely not least the Void Staff, with that extra spell pen and lots of ability power this with a rabadon's is very powerful.

for the Runes i went with the following:
Greater Mark of Magic Penetration X9 for the Magic Penetration.
Greater Seal of Armor X9 for that extra defense.
Greater Glyph of Scaling Magic Resist X9 for the extra Magic resist.
Greater Quintessence of Ability Power X3 for some ability Power
these are all good for a defensive Nunu but if you wish to have more ability power then feel free to change them to the following:
Greater Mark of Scaling Ability Power X9 more ability power
Greater Seal of Scaling Mana Regeneration X9 More Mana Regen just because Nunu is so Mana intensive.
Greater Glyph of Scaling Ability Power X9 More Ability power :D
Greater Quintessence of Ability Power X3 More Ability Power

for my Masteries i just went the best i could with adding in ability power and some extras to try and balance it out.

for the skill sequence i stopped at level 7 you may continue as you think what would be best.

i tried my best with this build my name is Telina and i play with the ranked team ATKGamers always in 3v3's but this guide is meant for both 3v3 and 5v5. only because it is a pretty expensive build as well.