This is how I play Nocturne. This is not Godmode nor is this the best build out there. This is meant to be a guide to smoothly transition between laning and jungling. Being able to switch between both mid/late game. That said, I'm not level 30 so I can't accurately test this build fully. Lastly, I never follow this word for word. Things can be changed depending on the enemy team. Let's get started killing!

Pros!

Easy 1v1 kills
Strong chasing power
Close to global ult
Spell shield
Easy transitions between jungling and laning
Strong turret pusher

Cons!

Early game mana problems
CC shuts him down
Squishy
One escape technique
Small use in teamfights

Nocturne has an odd set of abilities that all combo nicely with each other.

Passive

Umbra Blades... This skill thrives on attack speed and attack damage. It also helps you jungle well and stay in lane a little bit better.

Q skill

Duskbringer...Your main skill. It lets you rush in, escape, tank turrets, chase like a mofo and, generally be amazing. It has a built in Ghost and it only gets better as it levels.

W skill

Shroud of Darkness...Combines well with your passive with the attack speed boost it gives and becomes a nice spell shield. In my opinion though, it's never up when I need it to be. Helps you chase champions like, Cho'Gath, Sion and, Taric. But you need a bit of CDR to make chasing them truly effective.

E skill

Unspeakable Horror...More of your chasing skills. It also helps in teamfights if you land this on a carry or tank. The fear portion of the skill also helps in escaping.

Ultimate

Paranoia...It makes you the king of ganking at later levels. It also helps lead sneak attacks on turrets. Combo it with good timing, Shen's Ult and Twisted Fate's Ult and you have almost a guaranteed kill. It's usefulness is severely diminished in teamfights. It's better near the end of them when there are a couple people escaping.

All his skills are best 1v1. Some 1v2 scenarios can come out in your favor but usually end up 0v1.

18/0/12 is what I use. He needs the extra CDR and attack speed. I would go 21/0/9 but I couldn't find anything else in the offence tree that he really needs. He needs the gold increase so you can really get a jump on the higher cost items.

I don't understand why everyone feels the need to go twenty-one in one mastery tree and nine in another. It just baffles me. There are so much better combos if you go a weird hybrid of trees.

I'm not really good with explaining this but most of this goes with DPS characters.
Moving on!

Smite, taken obviously for the jungling aspect that Nocturne does so well.
Ghost, for when you're silenced or Duskbringer is on CD.

Alternatives:
Flash, for getting out of awkward spots.
Teleport, for super ganking power.
Cleanse, because CC hits hard.
Rally, for more pushing

What not to take:
Fortify...Really?
You won't need it late game.

This is a very expensive build with 2 items requiring a B.F. Sword but, trust me, it's worth it.

Leave the base with a Vampiric Scepter and head to the wraith camp. You can buy a health potion but you won't be jungling too long and you probably won't need it. On your first trip back, upgrade into an Emblem of Valour Emblem of Valour and buy a Meki Pendant if possible.

You want to bee jumping back and forth between base and lane as much as possible to buy all the stuff you need. After Tear of the Goddess, Manamune should be your next priority.

The Bloodthirster and The Black Cleaver can be interchanged but should be gotten one after the other. Boots aren't really needed until later levels due to the speed boost you get from Duskbringer but can be taken if you want to jungle sooner.

Your last item is seriously situational.

A couple general items you can take are:
Another The Bloodthirster.
A Tiamat because the splash triggers life steal.
A Phantom Dancer.
A Banshee's Veil.
A Wriggle's Lantern. The passive triggers life steal.

Specific Items:
For casters/mana users
For tanks.
For dodge users and tanks to a lesser extent.
Heal for more and life steal for more? I'm in!
Not much more to say about the items.

There are 2 sets of defensive runes because if he gets shot, he gets shot hard. If you can find better runes, by all means, use them.

It's roughly a 50/50 split with the Marks. He needs the speed boost a little and the ArmPen marks help him jungle just a little easier.

The Quintessences are just for the damage. Almost all his skills scale with bonus AD. They scale with pretty good ratios too.

Not much more to say here.
If you have better suggestions, tell me please.

Early/Beginning Game

You are going to be jungling a bit early game but it's going to be a very short route. Start at the Wraith camp and wait for them to spawn. Smite the big Wraith and launch Duskbringer so it hits as many of the smaller Wraiths as possible.

Run to the wolves after this and level. Drop your point into Unspeakable Horror and use it on one of the smaller wolves.

Run down to the golems and only focus on killing one of them. Smite the one with lowest health when smite is up again then run or recall back to base.

Later Game

Whenever possible, run to a small mob and take them out. If you feel like you're falling behind in level, jungle around.

Harass with Duskbringer and if you see an opening where an enemy is alone, take the opportunity and take them out. After level 6, you really should not be in one lane for far too long. Using ghost and Duskbringer to quickly get in between lanes.

If you're switching between lanes, keep a finger near your Ult button so you can jump on a gank if the situation arises.

Nocturne's idea of farming is launching Duskbringer into a group of minions, running in and letting auto attacks do the rest.

That's all there is to it!
Moving on.

Stay away from fights where they outnumber your team by 2 or more. Keep your eyes open for ganks from both teams. This is more feasible in 5v5s but it can work in 3v3s as well. Stay safe, assassinate well and don't forget, you can take a hit from Ashe's Enchanted Crystal Arrow with your spell shield up. If you get too ambitious will killing, you will die a lot!

Check out my other guides, here, here, here and, here. This section was made up so I could get within the 5000 character "minimum".