Pretty standard pathing, don't use your Q if you're full health, use your W to clear group camps like wolves and raptors. If you're gonna pass by potential enemy wards, make sure to pop the oracle lens.
Controlling the scuttle is key because it will not only give you a speed boost when ganking with your Snowball, but will also mean that enemies can't see your gank. Nunu is great for contesting the scuttle because of your Q, which acts like a second, more powerful, smite.

The predators are the key to your success. The most tried and true method of ganking is popping the predator boots around where the scuttle crab goes when it dies, then activating your snowball and rolling into lane. (Be carreful, if the scuttle crab is alive he will stop your snowball if you hit him, so be wary of that).
When you do successfully enter the lane, immediately start using E to slow the enemy, wether you hit your W or not. If they're not quite within root range, you can use your chilling smite to slow them even more.
Once rooted, your laner should be able to help you end the enemy's sorry life, with the help of your Q

Whilst it is tempting to use your Q straight away after a gank, you must refrain from spamming it. First focus on slowing them with your E, and then use your Q to either finish them, or gain any health they may have chipped off you during the fight.

When using your Q, ask these questions and respond accordingly:
1) Are you taking potentially fatal damage?
2) Will your Q deal enough damage to kill the target before they escape/kill someone else?

If the answer to either of these is yes, use your Q. If not, don't. There is no point chomping someone if you might need it more in 10 seconds. The same goes for jungle monsters.

When fighting an objective such as Drake, Rift Herald or Baron, your Q could be the one factor deciding if you win it or not. Hover over your Q to make sure you know exactly how much damage it does before engaging the fight on one of these monsters, then add your smite damage to this. If the combined damage is (for example) 2000, then you should make sure you can use your Q when the objective is at 2000 health, and then Q+smite it. Q not up yet? Kite the monster around or auto attack it until you're ready.
Remember! Your team is relying on you to get the objectives!

Nunu excels at soloing objective monsters all by his lonesome, so make sure to play to that throughout the game. See their jungler ganking up top? Take the drake. Top lane winning and ready to help if you need it? Take the Rift Herald. This is what makes nunu such a good champ, and don't ever forget that.

This is one of the best parts about AP Nunu. He is an absoloute ****house for flying into grouped up enemies. Your snowball can hit multiple people at once, so if you see them grouped up in mid lane (also known as the infamous 20 min ARAM) you know what to do. Pop those predators and start rolling.
As mentioned before, nunu can also fly onto Baron and either steal it with relative ease, or scare the enemies into forgetting about Baron, letting your team come in to aid you. As long as you can use those Predator boots, no-one can stop you.