This is the Nasus i prefer to play (In Ranked- and Normal Games).
I play him as an AD Offtank, due to in my opinion it's the best way to rush through all enemies.
I hope I can help you with this guide.
Much fun with Nasus.

The Summonerspells can be changed by every Summoner itself. I prefer Ghost and Cleanse.
Why?
Ghost : I play Nasus as a Solo Top Champion. At early ganks it's the best way to get First Blood if you are faster than your enemy. In my opinion it's the best Spell for Offense Actions or to Escape in a nearly lost Teamfight.

Cleanse : The most Champions in League of Legends are Crwod Control Monsters with noobish On Click CC (Like Sion's Stun, Rammus' Taunt or Skarners Ultimate). Cleanse is a blessing in 90% of all Teamfights.

I start my Game with a Philosoper's Stone and buy as soon as possible a Heart of Gold. Both let me stay a very long time on my lane and give me much gold per sec. When you go back you can buy full boots and the first part of your Trinity Force. I start this with Sheen, because it nearly doubles the effect of my Q Skill, let me creep much more and let no chances to the enemy in a good gank. When you have enoguh Gold you can go back to buy Trinity Force. Don't sell your gold Items.!
You build a Randuin's Omen out of Heart of Gold and next you buy Hextechdrinker. The bonus Shield of this (or Maw of Mormotius) can safe your *** or turn the Teamfight.

The last two items are your choice and should be choosen with the Game. You can go more tanky with Force of the Nature or a Second Life.
Or you can do it like me and build Warmogs+Athmas ( An totaly OP combo).

No matter how you choose, you will do a lot of Damage and won't go down so fast.

If you want to deal Splash damage : Tiamat is a very good Item if you have a huge stack on your Q. So every enemy get's hitten by the Powerful Stick!

Glyphes : I choose CD Reduction, so I can Spam my Q. More Q -> More Farm -> More damage.

Seals : If you want to spam Nasus' Q you need much Mana. So i have choosen Mana Reg. Runes.
This runes let you stay a long time on your lane (In Combinition with Philosopher's Stone).

Marks and Quintessences :
Armor Penetration! With this runes Nasus ****s up everyone. And squishie Champions are nearly onehits in lategame.

Your Main Skill is your Q. With this you have to farm a lot to stack up the passive AD Bouns.
This skill should be full stacked on lvl. 9.
Your W skill is Nasus OP Slow. You shloud skill it directly after your Q, because it's very usefull for ganks and to counter enemies while they are ganking.
Spirit Fire is a must have in every Teamfight. The Passive Armor Reduction let you and your Ad Carries with armor Penetration nearly deal True damage. Even a Rammus with Thornmail and Warmog's will be slained in seconds.
Fury of the Sands : Let Nasus Rise! With a good stack on Q and your Ulti you can win a 2v1 or even a 3v1. Bonus Healt and Passive Health absorption let Nasus be indestructible.

Your Passive: Bonus Lifesteal. A dream of every Carry. With this Passive you are able to survive a minority Teamfight and even get an Ace.

Nasus is huge Farming Machine. And every Minion you slain should be lasthitted by your Q.
Per Q lasthit you gain +3 AD on your Q. (In late game you have 200-300 creeps ;) )

Just remember a few things to play a good Nasus :

Farm with your Q.

Set your E under all Teamfights.

Start Ganks with your Slow.

If you remember this 3 points you will dominate the Game with the Curator of the Sands.