Hello and welcome to my first ever guide! This guide is for the champion Nasus, but im sure you knew that or you wouldnt be here. But what it specifically builds upon is his natural tankyness and throws in a lot of damage, what it focuses upon is his natural armour reduction from Spirit Fire and the physical damage done by his Siphoning strike.
This is in NO-WAY a guide that should be followed exactly and should only be used as basic guidelines of use and is what i find most effective when i play nasus in the certain premade 5vs5 ranked games (not very many but i have won every single game so far) and solo queue games I have played Nasus in.

Quintessences:

1 Dodge
1 Flat hp
1 Armour

This is my basic quintessence setup as I find it well rounded but feel free to exchange whatever you like freely.

Seals:

9 Dodge

I would say no question for this one, flat armour may be another choice but in my opinion completely dodging attacks is better than an (at most) 10% damage reduction overall and if you should choose to get dodge boots stacks up extremely well plus the dodge from the quintessence(s) will add up will make you fearsome versus any form of AD Carry.

Marks:

9 Armour penetration

This is also another no-brainer, as the build is based upon reducing as much armour on enemies as possible to do maximum damage. These will build nicely upon your other armour reduction effects.

Glyphs:

9 Magic resistance (per level)
9 Magic Resistance

With Glyphs you can choose to use either Flat magic resistance or per level runes. I prefer the per level ones myself as the extra bonus will really help past level 6. You will level pretty fast anyway because this build is meant for solo lane (plus you have the Awareness Mastery as well so you will reach level 6+ pretty fast mostly negating the early game bonus you will get.

0/21/9
For any sort of item build you have I would consider this important.
A lot of damage reduction plus the +2% dodge mastery AND the +10% movement speed bonus every time you dodge will make you a pretty speedy nasus (especially if you have all of the dodge runes on that i have suggested)

Improved Teleport and Improved Ghost will allow you to stay in lane and make you an even greater threat because of your ability to chase down enemies that stray too far from a tower.
Awareness will give you that much needed xp bonus to allow you to out level a 2vs1 lane even more effectively or to just keep a higher pace at gaining xp in a 2vs2 lane.

1 point in Spirit Fire at level 2 to begin farming and harassing early on,
maxing Siphoning Strike first to ensure as low cooldown as possible as last hitting with it is the highest priority for you.
Spirit Fire is then maxed out second as its armour reduction synergises well with this build as it is more effective than wither is early game in my opinion.
An early point of Wither is taken as its an important skill to have but not to max during the laning phases of the game in my opinion.

IMPORTANT: Last hit with Siphoning Strike as much as possible, dont just go smashing down your Spirit Fire till everything is 1 hit from death and then getting 1 of 6 kills with Siphoning Strike, The whole point of early game Nasus, once again in my own opinion, is to farm. People might call you a noob or whatever for staying in your lane for so long and pushing hard when people leave, but you know what? the second you leave that lane your gonna be farmed as hell and have your q doing more damage than most of what they can throw at you!

Generally after finishing Trinity Force you should either grab some tanky items if your having trouble such as an Aegis of The Legion (if the main tank doesn't have one ofc) or a Banshee's Veil or anything along those lines depending on the enemy setup.
If everything is going fine and your doing your job well go for a Black Cleaver to increase the amount of armour you reduce for your team. Then head into madreds Bloodrazor as it's surprisingly effective on Nasus then either built some more offensive items of your choice or you can start building a few defensive items like the ones listed above.
Boots have been left out in the main item build up top as ive seen many viable nasus builds with many kinds of boots, you can choose wahtever pair you like but generally I will take the Merc treads or Ninja Tabi.

Pros
Awesome lategame
Extremely tanky after phage+
Powerful slow to close gaps with those squishies that think their safe outside tower range (its always satisfying watching them try to harass you and you just slow, run up and then smash them right in their smug little faces)

Cons
Sort of slow start even though the pace picks up after you buy sheen
Dependant on farm with Siphoning strike

Overall i hope you enjoyed reading my guide as much as i enjoyed writing it and i hope it helps you get better at Nasus, as i never see any and if i do they are always pretty terrible so hopefully i can change that!

The entire trinity force tank idea was taken from:
http://www.mobafire.com/league-of-legends/build/offtanks-the-new-metagame-79701
and my build was centred around becoming the new tanky meta-game champion explained in this guide, i advise everyone to take a good look at it as it is very informative and well written.

Thanks for reading and please do not hesitate to be harsh in comments as its my first guide and i need some pointers. ^^