This is the way I play Nasus the Curator of the Sands and it has worked wonders for me.
I'd apreciate feedback on this and it would be nice to know if you have succeeded with this build.

Basics of this build is that you last hit with your Siphoning strike. Other last hits are of course important but that's your priority and you should save your Q cd to last hit and power it up in that way. Every last hit increases your Siphoning Strikes damage by additional 3 and with Sheen proc this starts to stack up into high damage hits pretty quickly.

Nasus is slow as hell so one movement speed rune really helps him. Otherwise runes just are aimed to be little tanky and with arp runes to really hurt something. If you want you can swap your armor per level quins with health ones.

This mastery build gives you decent damage early and makes you little bit more durable. With Evasion Seals and Nimbleness you get little early game speed buffs too.

Level up Siphoning Strike first and pick one level in Wither at level 2 then let it be until level 10 after that level it to max rank.
Take ultimate when ever you can and Soul Fire at levels 4, 8 and after that level it last.

This sequence gives you good early damage and farm but also makes you decently powerful in mid game skirmishes.

Remember your first priority with farm is to power up your Siphoning Strike. With decent farm on it early and Sheen your farming comes even easier and you can even use it to harass in a way that hurts.

I start with Sapphire crystal and 2 HP pots. This gives me mana to spam Q and also cast other abilities if needed and still have easy way to regen with passive lifesteal it's easy to stay lane if you play carefully.

Core items to this build are Boots of Swiftness, Trinity Force and Sunfire Cape. These tree items give you dmg and survivability so when you go out there and stand amongst your opponents with ultimate activated you will dish out pretty decent amount with ultimates tic and Sunfire Capes aura and can take pounding too.

Rest of items just increase your damage and survivability. You can swap Madred's into Black Cleaver if there's high armor targets against you. There is no mr items because AP dmg dealers are usually squishy and really don't want fight with you and you should target those first and after that target AD champs. Frozen Heart is a must with it's CDR and mana, these make your Q really spammable.

If there's loads of AD damage coming at you and/or your team lacks good fight initiation you should build Frozen Heart before Bloodthirster.

As summoner spells I recommend Ghost and Ignite.
I pick Ghost because it's really versatile movement spell and gives you good chasing or good escaping tool.
Ignite helps you get those early kills more easily and late game it makes great tool to bring down those tough and high HP regen targets (e.g Dr. Mundo or Warwick).

+High dmg output
+Late game really hard to kill
+Gives your team off tanky champ
+Can deal high dmg to turrets if needed
+Great dueler

-Requires a lot of farm with Q
-Expensive items
-After few fights you'll be high priority target

As stated before you must get last hits with your Q and make it more powerful. This isn't too hard but even as come goes further you must still farm. In 30-40 min game you should have over 300 bonus damage with that.
When going into team fight you should wait until fight has started properly and then go in so you won't get focused right away. Pop your Soul fire under fight activate your ultimate and start spamming your Siphoning Strike. Remember Soul fire also helps rest of your team because it reduces armor of the enemies standing on it.

When dueling against champions you should lead with wither, use Q when ever you can and wait before you use Soul Fire. You should use Soul fire when your opponent stops, usually after wither ends, and pop it under your fight. They usually won't understand the danger Soul fire brings after all it's damage isn't all that great.
So dueling is simple. Use Wither and Siphoning strike when ever they are off CD and Soul fire if the fight is stationary. DO NOT use your ultimate if you can win without it.

I have had loads of fun with this build and with decent team you will just march over everything.
If there is already killing power in your team already you can also take role of pusher with this build and you'll be rather safe, not many can duel with you. Just remember Siphoning Strike also damages towers.

Give me feedback how this worked for you and enjoy!