Well Nasus is a strong Champion and he is very useful in teamfights.

His slow is in my opinion the best in the whole game and his Ultimate is slowly but surely stealing life from his enemy.
I always just try to survive until lv 6,playing super safe trying to stack my Q:Siphoning Strike and not waste my W:Wither,until my Jungler comes to help me or the enemie´s jungler comes to help my opponent and so i can save myself even without using Ghost.
Well but if you see that your opponent has no flash and you feel sure and have some movement you can use your W spell to catch him.I always w8 to get at least half of my built ready and then go into teamfights.But then i dont have any problems.You are the tank of the team.Your task is to slow and lower the Magic Resistance of the enemies with Spirit Fire .
And thanks to your passive Soul Eater you have nearly no Lifeproblems,sometimes a bit mana problems but pots can help you. And you wont have any problems in mid and endgame.

The enigmatic creature known as Nasus came to the League from a faraway world, and served as part of a vast and learned desert empire. There he was a respected and enlightened individual, having been made the Keeper of the Great Library. However, he was also part of a race of animalistic creatures that served as the rulers and protectors of the people. Unfortunately, there were others of his kind that believed they were more than protectors. They were under the egotistical delusion that they had ascended to become gods. War on Nasus's world was never more than a breath away, as these deluded others sought to enslave the people and establish a totalitarian sovereignty. In the midst of a heated battle, just as Nasus was about to slay his treacherous brother Renekton, he was pulled through the cosmos to a summoning circle in Runeterra.

At first angered that he had been taken just as he was about to cripple the enemies of his people, Nasus' advanced intellect won out when he heard the pleas of mercy from the summoners. He learned that Runeterra was also filled with its own brand of injustice. This danger was no less threatening than what he had fought against at home, and champions were needed to protect the people of this world. Heeding the call, Nasus chose to take his place in the League of Legends, knowing that his brothers and sisters would carry on the fight back home. These days, he is the Curator of the Sands, feeling most at home in the crumbling ruins of the Shurima Desert. However, recent disturbing news has reached the Keeper's ears - rumors that perhaps his brother had not in fact been left behind. If this were the case, the destruction of Renekton would quickly become Nasus' main priority, before this world too faces the threads of his brother's forked-tongue and endless ambition.

The Curator of the Sands, though contemplative, is not to be trifled with

The runes give the perfect mix of a Tank and give you a good raise on your Auto Attack Damage.
You can swap the AD Runes against Armor Runes but i think the item will give enough armor.

I have taken this Masteries because you still have the task to tank in games.With this built your opponents need to use more abilities which cost energy,mana or rage and if you can kite them to your team,your mates have really a easy job.And if someone has the cheek to attack you well then you can be sure you win the battle.With that built you slay them in a 3on1(but do not forget to use your ultimate Fury of the Sands,where you gain 300/450/600 Health and deal 3/4/5 (+0.01)% of nearby enemies' maximum Health each second (240 damage max per second) as magic damage and converts it into bonus Attack Damage.

NOTE PLEASE: often games dont last so long that you can buy your full built.

This built is rather expensive with items like Trinity,Warmogs and Guardian Angel.
But this built gurantees that no one will focus you in 5on5 Teamfights and you can be the one who slaughter them.

Well you must skill first your Q spell:Siphoning Strike.

With this spell if your opponent is in range you start the battle,otherwise use W and E first if you are not in range.
At level 18 i do about 900 to 1200 damage with activated Q on enemies who are not so tanky.
And well the little but fine extra damage of E:Spirit Fire helps you to slay who ever you like.

I love Ignite.

The damage is nice but not the reason why it is my favourite spell;i like the effect of halfing the enemie´s heal and healreggen.
Thats why you can slay Mundo,Swain or others with a Warmogs still quite easy.
Ghost is nice to chase or escape or just to run fast to the the point of map you want.Teleport is nice too but has really high CD ( cool down) and well flash can be used too but since the nerv it is not the no 1 spell anymore. I think exhaust is a great spell but not for Nasus,it is simply wasted cause Nasus has the best slow of this game.

I play this in Ranked games and i never lose my top lane and i never die more then 3 times a game and often i make at least 10 kills every game.
Nasus if played right can bring you out of Elo hell or raise your good Elo even more.

And Nasus do not get banned hihihi

PRO:

+this built makes Nasus unbeatable
+You will score triple,quadras and even some Pentas
+his passive is the reason why you wont have life problems
+best slow of whole game
+Egypt fans and guys who have a Dog should play him xDDD
+who ever is good with placing spells has no problems too with NASUS

CONS:

-you need to be good with spell placing should think it over to play Nasus cause the E spell is a very important spell,even though compared to the other spells it has not such a high meaning for the game.
-with this built you have to play safe until you get your ulti and built if the opponent has a flash or ghost or fast jungler
-very expensive built,but it is a good one but dont look at it if you are not passion enough to play this.

Farming is the most important for Nasus.

Try to walk all the time at your lane and so be always faster than your opponent and farm and farm and farm with your Q spell. I often press E to speed up the farming.

If you want you can buy a Frozen Mallet instead of Trinity or Tiamat instead of guardian angel but it depends from situation to situation but the first built is really Numero Uno

Have fun with this guide,rate,comment but before you do this TRY IT.