THIS GUIDE IS STILL UNDER CONSTRUCTION! NOT DONE BUT THE MAIN STUFF ARE HERE EDI! ;P
Hope you don't mind.

To tell you guys the true, I don't intend to show you all such super detail builds or guides. I'm here just to tell you all the things that I think you need to know for using this hero. I too don't intend showing off my recent match history or how people think this build help them a lot or not. I really think it is pointless, because when ever I use someones guide,I only see their builds and what they buy and stuffs. So why bother...:) The main objective is too know what to do with Nasus during a game.
Anyways, in my opinion, Nasus is actually one of the best heroes out. Like I said,my opinion. In this build that I've been using,My KD(kill/death) has always been positive(as in,I kill more then my deaths,for example,12/1)
If you like tanky DPS whom can slow,have life steal,have skill that can go up to 1k dmg per 2.4 secs with the right build,Aoe skills,THEN I HIGHLY RECOMMEND THIS BUILD.
Short story long,I'm only talking this much because because the minimum words is 5000 so yea :)

You can go to the cheat sheet to check on the rune build but right now i just one to touch on the reasons for my runes...Greater Mark of DesolationGreater Mark of Desolation
This is probably the most important rune for Nasus to have! Using this rune helps you with your Q which is Siphoning Strike . I'm pretty sure every Nasus players uses this rune. The ones that probably don't are Noobs that would give this up for some other rune like Greater Mark of Ability Power which is really not necessary.
Greater Seal of VitalityGreater Seal of Vitality
This rune isn't really necessary but from my pass experience with Nasus, I always need just 1 more hit to kill the last enemy but just don't have at least 50 life to do so. But another suggestion of seal runes to replace this one is Greater Seal of Replenishment. It really helps at beginning game and you don't have to waste your money to buy Chalice of Harmony.
Greater Glyph of Celerity
Using this rune is really helpful because in this build,My main skill is my Q Siphoning Strike. Being able to use Q as your normal attack gives you a lot of advantage(because the lowest dmg you can do in lvl 18 is 600,imagine that,and like I said,LOWEST which means some noob unable to farm his Q ) This rune helps your W Wither too because you will be able to slow your enemies more frequently which help chasing or retreating.
Greater Quintessence of Swiftness
Greater Quintessence of Movement Speed helps you chase,run away,last hit your Q easily. Just that simple, Nasus is great with movement speed.

Some people just do not get this runes,It may seem like a joke but once you try it,I'll be the last one laughing.:)

1st of all!!! I just want to say don't buy Doran's Shield! Please,it hurt my heart seeing you waste your money.
I would usually start with Doran's Blade, because Doran's Blade too gives you health, and not only that, you get damage and life steal. You will be able to lane without going back and heal yourself. Every time I play,I'm able to lane with my enemies for a long period of time. Which give an upper advantage, and by the time I go back to base, I have about 1.8k which ables me to buy a Boots and Sheen. At this stage of time, maybe say 9 mins,your Q will at least do 300+ damage every few seconds.
After that,straight aim Ionian Boots of Lucidity,it is really important. I think you should know why. So on,you'll get Phage then Trinity Force( Trinity Force is probably the most important item for Nasus,Only stupid DPS Nasus don't get this!).
By now,you'll be able to kill quite alot but that's not enough. You will be needing Atma's Impaler. FYI:I REALLY HATE PEOPLE SAYING ATMA IS USELESS OR A STUPID ITEM BUT BELIEVE ME,IDIOTS ONLY SAY THAT BECAUSE THEY ARE THE PEOPLE WHO BUYS STRAIGHT DAMAGE. Buying Atma's Impaler give you armor,critical chance and also damage! Your attack will almost be 200.[Not to self,don buy buy this item when your life is below 1.8k,maybe buy Giant's Belt 1st]. After that,buy Warmog's Armor then followed by Frozen Heart. Last item,it will always depends on the enemies heroes,but mostly,you'll need to buy Banshee's Veil. Last and second last item is always optional to my opinion.

I know my Masteries seems weird,in fact it is weird. I really don't know how to explain my masteries ,but I believe it is awesome. You might notice me getting the spell Ghost and not get the masteries for it. This is because you need Archaic Knowledge which is more important. Besides,I have set the runes for it. But that doesn't you should not care about your movement speed.
By getting the full item build,your movement speed will be 452,which is better than average. And imagine getting ghost. And you too can slow your enemy. ;) Enough said.IF YOU ARE JUNGLING.
http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=3-0-1-0-4-0-0-1-0-0-0-0-0-0&tree2=0-3-0-3-3-0-2-0-2-4-0-3-0-1&tree3=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0

STRONGLY RECOMMEND!
GHOST-your a ghost and you can chase! :P
FLASH-If you're not a big fan of ghost,then you can get flash.
POSSIBLE TAKES!
EXHAUST-Slow,and stuffs. Good for fighting DPS
TELEPORT- Helps you to farm easily,especially when you are low on health/mana,u can just recall then tele back .
SMITE-If you want to jungle with Nasus,I don mind. I've tried it but I rather lane.
CLARITY- Nasus has a big problem with mana. Unless you mostly play your Q. And also since this build doesn't buy Chalice of Harmony
SPELLS I RATHER NOT TAKE
IGNITE-Since you have all that damage,you really don't need ignite unless you have problem with last hit.
HEAL-Since you have your Q,you can heal. Especially since you have life steal for your passive.
CLEANSE-Your not a tank who engages and let everyone attack you so no need.But take if you may,I just don't recommend it.
FORTIFY- Hmmm,you can get this if you want but it only means your weakness is out ranged of your turret.
RALLY-Not so good for Nasus,maybe some other heroes but not Nasus.
SPELLS TO IGNORE
REVIVE-I really believe this spells is ****py,but that's my opinion

Don't really have anything to say. This build is for this skill sequence. :)

OVER ALL!
The important points you need to know.

-Armor Pen Marks are IMPORTANT and a MUST GET

-Follow my masteries because it will give you CD,magic pen,and other stuffs.

- Trinity Force and Ionian Boots of Lucidity is a MUST. Atma's Impaler and Warmog's Impaler is a good combination and Frozen Heart helps you chase and CD.

-Try to max your Q as fast as possible and your W after that. Just follow the order.

-Always play with your Q! Most important skill.

-Oh yea. always use your R when your life is 3/4. Don't just straight use it.

-Another thing,I don't know why but the stat up there says 146 damage but actually it is around 250 and life steal is around 20%