This is one of my first builds so be gentle. :D

Nasus is one of my favorite champions because he is able to deal damage while getting no damage items like skarner and udyr, besides sheen/trinity force. He is able to do this because of his Q which stacks damage and can do an infinite amount of damage depending on how you farm. This build is to make him have alot of tankiness with sustainability. The best way to use this build is if you are moderately good at getting last hits, such as getting at least 50 minion kills at 10 minutes. This build should give 40% CDR to max it out for optimum farming.

Passive: Soul Eater

This skill gives passive lifesteal which makes your sustainability in lane very high.
Key Q: Siphoning Strike

This is your best skill it is what makes nasus well... nasus. For every minion,monster,champion that you last hit with this skill it adds +3 damage to your Q. That means that you can build it infinitely, making your damage output practically unstoppable like veigars.
Key W: Wither

This is your skill that slows down an enemy's movement and attack speed and builds up per second. Basically it slows them down to the point they can almost not move or attack. Use it to catch up to an enemy and pretty much negate the other teams attack damage carry.
Key E: Spirit Fire

This skill is an AoE debuff and damage which lowers their armor and on the area casted it does a bit of damage which can be used to finish off enemies or just deal a little bit of damage over time. Use it under the area you are fighting on.
Key R: Fury of the Sands

Nasus' very tanky ability which many people underestimate when fighting against him. It grants him a substantial amount of health and makes him do a percentage of their maximum health each second. Use this to save yourself or to initiate and tank.

Greater Mark of DesolationMarks: The usual flat armor or armor penetration depending on if you want to tank
Glyphs: Magic resist or cool down reduction helps tank and farm
Seals: Armor or mana regen per 5 work for tankiness or sustainability
Quintessences: Health, armor, or armor pen depending on how you play. I go flat armor for tankiness against melee.

Starting items: The Regrowth Pendant for health regen and to build it into a Philosopher's stone. Doran's Shield for health, armor and health regen. Ruby crystal for health and to build it into a Heart of Gold.
Early game items: Philosopher's stone The P. Stone helps with health and mana regeneration as well as gold per 5. Heart of gold for health for health and gold per 5. You choose between Ninja Tabi for it's low cost, armor, and damage reduction to Mercury's Treads for it's magic resist and tenacity which reduces all forms of crowd control. Depending on which type of team you are facing (Melee or Magic) that is what you would usually get. Glacial Shroud for the cool down reduction, mana, and armor.
Mid game items: The Frozen Heart for cool down reduction, mana, and armor. Sheen for mana, ability power, and the fact that it increases your next auto-attack by your attack damage on spell cast, which means that when you use siphoning strike it hits even harder. Shurelya's for the health, cool down reduction, and its active, allowing you and your teamates to move faster allowing you to catch up or run away from enemies. Randuin's for the armor, health regen, health, cooldown reduction, and its active/passive, slowing enemies movement and attack speed, helping you slow down enemies to catch up to them or run away from them.
Late game items: These will be your final items. The Trinity force for the extra damage, movement speed, and health, this will be your bread and butter for damage output. Now for your last item you can choose from almost any item, but in my opinion the best four are Force of Nature, Banshee's Veil, Thornmail, and Guardian Angel. All of those add to your role as a tank. FoN and Banshee's are for magic damage heavy teams, Thornmail is for attack damage heavy teams, and Guardian's Angel is for balanced teams or if you want to get back up for that pentakill.

These are the summoner spells I recommend, the other spells. Just no.
This is for slowing down enemies and practically negating their damage for a couple of seconds. Would only use this if going against a melee top.
This is for reducing an enemies healing and finishing them off. Would use this if facing a champion that always seems to get away.
This is for teleporting around the map or back to your lane. My favorite summoner spell for solotop as if I am facing 1v2 and I need to back, I just teleport back once I'm done. It also should be used to help teamates turn a 2v2 or 2v3 fight to 3v2 or 3v3 fight quickly. A trick is to place wards in bushes or hidden from enemies which you can use to teleport to.
This is to blink away, over walls, or towards and enemy. This is very useful to escape or to get closer to an enemy to wither them or finish them off.
This is to sprint towards or away from an enemy to catch up to them or dip out to avoid dying. Very nice when ganking and when an enemy champion sees a tank steamrolling towards them, they have no choice but to run.
This is used to heal yourself and teamates. I would not normally use this summoner spell unless I'm playing support, but hey, if you want to and your good with heal or not sustaining yourself, go ahead.
This is used to remove various forms of crowd control from yourself. (does not remove malzahar and skarner's ultimates as far as i know) I would mainly use this only for crowd control heavy teams.

"The difference between a good nasus player and a bad nasus player is their ability to farm even when put at a disadvantage."-Epic Raid Boss

Must farm, must need, must win.
To farm well, do not attack the minions when they are at full health. Only when you are to finish them off or need to regenerate some life with your wonderful passive, that is also when you build up the stacks on your Siphoning strike.

If you have farmed well you should have at least 200 minion kills in a 30 to 40 minute game.