This build pretty much turns nasus into a viable tank/dps. With the extra health regeneration of a warmogs at full stack, and his dps abilities, nasus is a truly formidable opponent.

For runes i take a variety of offense and defense runes. each one catering to a specific situation. Glyph of vigor to help me early game to remain in the fight if trapped by an opponent. Works even better once your warmogs is complete. Seal of evasion, this runes come in handy when being chased at low health by an enemy to complete your escape alive. Mark of Furor, these runes really help in later game after you build your first critical % item. Mark of Desolation, these runes enable you to punch in that extra true damage and often enable you to pick up those extra kills off runners. Quints of evasion, same as before with the seal of evasion, avoiding damage is always good.

For my items i recommend beginning to farm with last hitting spirit fire on enemy creeps. This farming will allow you to quickly get the gold required for the Warmogs Armor. As soon as you get a warmogs continue as before farming as many creeps as possible with spirit fire and farming siphoning strike by last hitting nearby creeps, this will allow you to farm health from your warmogs. "if you are having trouble farming gold, either buy a gold per/5 such as philosophers stone, or get gold Quints"

Berserker grieves, great boots for nasus as it allows you the extra attack speed which raises the dps you can punch out. Also lets not forget the +2 speed, a must have for nasus to claim many kills later in the game. To often enemies will simply outrun you and have another enemy run up behind you while chasing and gank you. Trinity force, "edit" After advice from other members, and a few trial runs. It has come to my attention that gaining trinity force early really helps out in mid game. The addition of all the bonuses of trinity force does help get extra kills throughout "the game".
Onto Thornmail, optional depending on what kind of opposition you are up against. Are the enemy players mostly mages or physical? if physical go with the Thornmail, if magical i would choose a Banshees veil.

The addition of an infinity edge works well with the crit runes already mentioned.
Now is when i would really start to seek out ganks and chase enemies with some confidence. Never over extend the chase however, play smart not brave. Okay an onto the last item, the bloodthirster, late in the game your going to be pretty tanky. Bloodthirster allows you to do an extra bit of damage, and do better with nasus' natural life steal, helping you last longer in team fights and solo.

Leading with wither and following with siphoning strike is a great way to quickly damage your enemy. If able following with a spirit fire in the path the enemy will be fleeing in will help as well if they run low on health and cross it.

When in all out combat with another player do as i say, use wither, drop a Spirit fire on the enemy, use your q which you should be constantly farming by last hitting minions with it. As soon as you can get the previous combo out, pop your ultimate and if you have taken ignite use it as well. Focus from this point on in keeping your spirit fire and wither up. using siphoning strike when not doing so as much as possible. In late game this combination is highly deadly to most non tank type builds.

Nasus late game with almost 4k health is a very effective meat shield for turrets while allies drop said turret. If very low on health but just need a bit more health to pull it off simply pop your ultimate and you will gain health on cast and post end.

With the bit of variety in rune types listed in this build you are offered more variation in build type and use in battle.

when focusing on chasing opponents or playing a mainly running type game, ignite/exhaust are great spells. wither/ignite/attack/exhaust/attack/wither/attack

Self Explanatory

Nasus does very well on farming minions thanks to his AOE Spirit fire. Leveling that spell up to 5 and dripping on a wave of minions helps nasus gain gold very fast especially if in combination with a philosophers stone.

And speaking of farming FARM Q!!!!!!!!!!!! It is unacceptable complete a game with less than +200 damage on nasus' Q

Nasus has the ability to play the role of an extra off DPS or a tank depending on how he is built up. Or as this build has shown he can be a combination of both effectively.

Paying attention in team fights and knowing where to drop your spirit fire are skills learned from playing. Just always be sure to make use of siphoning strike and spirit fire in team fights.

Nasus late game is a force to be reckoned with however, early game careless play often ends in death. This often leads to scores which aren't quite as high as some would like. For this i would just advise playing it a bit safe and keeping minions in the middle of the lane not by either turret. Once sufficient dps and health has been accrued however you may enter GO/GO nasus mode. xD