In this build, Nasus will be jungling for the early stages of the game.
You'll want to start by getting a blue buff leash, or invade blue buff on the opposing team, if possible.

Your strongest attack is going to be your 1st ability "Siphon Strike". With the right timing and constant re-usage, you can already be on your way to turning yourself into a powerhouse. Your focus is going to be building up that attack to at least +75 extra damage (25 creep kills with Siphon Strike) before you get into team fights. This will ensure potential damage output early on, faster leveling, easy farming. Use smite whenever you can for the +10 gold buff that you should have from your masteries.

PROS: You're building yourself into an unstoppable adversary! You'll be the ultimate result of the opposing team's failure, and nothing is better than advocating the winning potential on your team. Played properly, your death will be minimal, and your kills will be phenomenal. Be a team player, and you will lead them to victory!

CONS: You're going to be a little bit squishy. You must keep your team in mind before yourself, in that you have no room to show off, and must play strategically. You are the project everyone will be working on in the game, in that you are the result of good team fighting. If you can rely on your team mates to be there for you until you're built like a brick-****-house, then this is the build for you.

Very simple.
Blue Buff Golem > CYCLE (Wolves, Wraiths, Golems) > CYCLE > CYCLE > Wolves > Wraiths > (Level 4-5) Red Buff > Golems > Wolves > Blue Buff > GANK

You should find no trouble in this cycle, with your siphon strike ready to dish out lots of damage, your smite for back up in sticky jungle situations early on, and innate life steal to keep you alive longer. By level 3, this should be simple rinse and repeat.

If one of your ally lanes is getting overrun, swap in when you get the chance to help them out if they need it, or take over a lane while they recall back to base.

By the end of this phase, you should be level 6-8, with Berserker's Greaves, Phage, and possibly Spirit Visage.

1st Ability - Should be powered up enough to dish out 300 some odd damage. Rock n00bs with this. Keep building it up as you play, remember to use it as your last hit, and you'll see it sky-rocket up to do over 700 damage by end game, if they don't surrender!

2nd Ability - This is a 34/40% Attack/Movement Speed DEBUFF. It is crazy. You should have no trouble tossing this on a run away opponent, or someone who wants to be a tough guy and chase after your allies. Save for necessity - great for initiating.

3rd Ability - This is a Damage over Time circle of ARMOR DEBUFF. Toss this up the moment an area of team fighting makes itself present - generally once someone initiates and everyone's in the circle, this will ensure someone will die, especially the squishies! So long as they fight in the circle, easy win!

4th - This will make you grow big. It adds a ton of health to you and grants you an aura that will cause damage to any opponent who is near you. Pop this baby when you're losing health rapidly, or when you are jumping into a team fight and want to help with crowd control.

The focus is stacking ungodly amounts of damage between your ability power, base attack and unique passive abilities. Hand in hand with attack speed, and the abilities you have as Nasus, you'll be a wrecking ball in team fights.

Berserker's Greaves > Attack speed + movement speed.

Phage > Extra Health, Attack Damage and a chance to slow enemies!

Spirit Visage > Cooldown Reduction! MORE HEALTH! OPTIMAL LIFESTEAL AND REGEN!

Sheen > Ability Power and MORE MANA! + passive stacked with 1st ability (which is PHYSICAL) = +100% EXTRA DAMAGE.

Zeal > Movement Speed, Critical Chance, Attack Speed.

Trinity Force > Boosts and optimizes everything we just bought prior to Trinity. You are officially a POWERHOUSE! Time to make you OP.

THE BLACK CLEAVER > Attack Speed, Damage, -45 Armor Debuff after 3 hits (Which makes them weaker against YOUR PHYSICAL DAMAGE!) This stacks well with 3rd Ability!

GUINSOO'S RAGEBLADE > MOAR Ability Power, Damage, plus now when you attack or cast a spell, you will increase your attack speed and ability power for a maximum of +32% / 40AP (stacks 8 times, so get in there and start whacking!)

WIT'S END > Attack Speed and Magic Resistance. Attacks have a +42 Magic Damage stacked on top of it, and you build extra magic resist the longer you're in the team battle. Build interchangeably with BLACK CLEAVER, depending upon mid-game situation.

You're officially OP. :3