I'm going over the skill's "skills"/skill order/"correct" way to use skills here.

Abilities

Soul Eater: Your passive which gives 15/20/25 life steal. It's not a great passive until you start to Siphoning Kill everything.


Siphoning Strike:Siphoning Strike is your best skill by far. If you are bad at last hitting, you will fail hard with this. It resets your auto attack timer so you can effectively last hit easily once you know you attack animation's speed. Doing this is crucial to learning to stomp with Nasus. It snowballs fast and effectively into late game. The bonus damage it gives your attacks, also works on turrets.

"Correct" Skill Usage

Siphoning Strike:

• Do last hit with it active.
• Do use it to quickly push turrets.
• Do use it in combination with Fury of the Sands or Spirit Fire if possible.
• Do use it whenever possible when fighting Baron/Dragon/Buffs
• Do use it to nuke down opponents.

• Don't use it just to slightly damage enemies other than champions or super minions.
• Don't trigger it unless you are coming to a minion line or camp with enemies you know you can one-shot.

Wither: Wither is your one and only form of CC. It and Spirit Fire will pop Banshee's Veil bubbles. If you hit people with this, it applies an initial slow to both move speed and attack speed along with an extra slow over a few seconds. Coupled with the speed you have off this build, people will not be able to escape. It also doesn't pull turret aggro so, it can buy you a few extra seconds to push a turret. Mercury's Treads severely kills the power of this skill due to the "slows over time" effect. If you use it on a person wearing them, be prepared for a faster retaliation.

"Correct" Skill Usage

Wither

• Do use it on a fleeing enemy.
• Do use it on an enemy to pop a Banshee's Veil bubble if your caster is readying up all their most powerful spells.

• I cannot stress this enough. Don't use it to initiate on Irelia or anyone with some form of CC reduction.
  
  There's not much you shouldn't do with Wither. Just don't randomly use it if you know you or a teammate can't get a kill off it. If you are coming out of the jungle with it queued up, it's fine to use it if they get away as long as you either have blue buff or they used a summoner spell to flee.
  
  
  
  
  Spirit Fire:A nice bush checker/stealth deterrent. Lowering armor of things standing on it is a nice bonus. Combo it with Siphoning Strike and your passive life steal, you have a nice heal over time. It does a deceptively good amount of damage. Sometimes netting you kills just for laying it down.
  
  

  "Correct" Skill Usage

  
  Spirit Fire

  Do's

  • Do use it to de-buff people during team fights.
  • Do use it to help you farming of Siphoning Strike
  • Do use it a slight stealth deterrent. (Saved a Sona with it a couple of times)
  • Do use it to check bushes.
  • Do use it in combination with Wither and Siphoning Strike for more damage.

  Don'ts

  • Don't use it as a form of harass.
  • Don't use it if you won't have enough mana for another skill.
  • Don't forget about it when stealth-able melee champions are involved.
  
  
  
  Fury of the Sands: You ultimate and the skill that really makes you fit into the Tank/DPS role. Giving you bonus HP and dealing a percentage of all nearby enemies' HP per second and adding to your AD for a while. This is not a skill to be taken lightly. Using it correctly could mean the difference between death and surviving a 5v1 gank. (This happened to me. I died because I turned around and got cocky.)
  
  Fury of the Sands

  Do's

  • Do use it in the middle of many enemies.
  • Do use it to escape surprises if needed. (Yes, 5v1 ganks are an acceptable surprise)

  Don'ts

  • Don't use it while fighting the "big" buffs unless you are "surprised". (Dragon/Baron)
  • Don't use it whenever it's off cool down.
  
  

  Skill order

  
  In ideal situations, you want your skill order to be Fury of the Sands> Siphoning Strike> Wither> Spirit Fire This will make sure that your ganks are as strong as possible. If you really need to just sit and farm in the jungle, or if a ton of team fights are breaking out, change your skill order to Fury of the Sands> Siphoning Strike> Spirit Fire> Wither Avoid doing this if at all possible.
  
  

  Copied directly from my older Nasus build. No way in hell was I going to re-type that whole useful section......

These are somewhat negotiable.
The armor pen Marks will put you at just the right amount of armor pen to deal true damage to all the jungle monsters with your Spirit Fire on them.

The AD Quints and Marks will give you the damage to stay alive during the route with your life steal. In order to have the absolute safest jungle, you have to have at least one AS quint of tier two or three. This will ensure that you don't have to be as exact with your potion usage and, you could possible have one left over at the end if done right.

Take Armor Seals or else you will not have enough HP to do anything.

You have free choice with the Glyphs. I take Greater Glyph of Magic Resist so I can be a slight bit tankier.
Suggested by jhoijhoi Greater Glyph of Cooldown Reduction may be used to bring your cooldown of your Ult to acceptable levels. It will also help early with farming of Siphoning Strike in the jungle.

You are the jungler. Make it a habit to buy wards even though it will take you down a bit of cash. I have a habit of buying a slot up to 4 wards as often as I can.

In the initial stages of the game, these are crucial. How you use them is equally important.
You have to get the Cloth Armor and 5 Health Potions.

As a "tank", your next item should be Aegis of the Legion. On the other hand, if people are just barely getting away from you, you might want to settle for boots first.

Boots are completely situational. You may either choose to be a roamer with Mobility Boots or lean more towards the tank side with Ninja Tabi or Mercury's Treads. You should not be getting Berserker's Greaves or Ionian Boots of Lucidity. You don't need them.

Moving on, you want to build Trinity Force as it, combined with Siphoning Strike will become your main source of damage.

After that, you have free reign of item choice. I usually build a Phantom Dancer for the move speed and the fact that I am regaining "more life" from my life steal.

After you've built your core that you see up top, you want to be building massively defensive. You are currently a glass cannon, albeit a powerful one once you start hitting things.

Banshee's Veil and a Guardian Angel are really your best bet. They can be substituted for more specific resistances if they are needed. Because of how fast you can rack up gold after you're done jungling, dropping something for a Thornmail or Force of Nature shouldn't be that hard. If you find yourself running out of mana later game, grab Frozen Heart because of how well it synergises with the rest of you.

Note: Due to the expensive nature of the build, you may only get to Trinity Force in the item sequence. Just, keep that in mind while you play this build.

Smite This is important! If you do not use this, you will be laughed at because of how badly you will jungle.
Exhaust This is your "grace" spell. This can be exchanged for anything you feel like. Because Wither acts like Exhaust it gives you a slight upper hand on ganks and chases. You may switch this out for any of the following as you see fit. (list in order of preference)

• Ignite
• Flash
• Cleanse
• Rally

The key section of the guide!!! Start at blue with Spirit Fire primed up. If you are getting a leash, you want to delay Spirit Fire's placement until he aggro's onto you. Otherwise, he may only get the armor reduction for a second. This de-buff is key to your jungle! As soon as the golem begins hitting you, use a health potion. If you don't do this, you will die. Use Spirit Fire as much as you can to keeps it enough armor for you to do true damage. Use smite when appropreate. Put your point in Siphoning Strike and move on to the wolves.

Last hit as many of them as possible with Siphoning Strike while continuing to use Spirit Fire to deal increased damage. Use a health potion when your first one wears off. Move to the Wraiths.

Stand in the middle of them before laying Spirit Fire down so you have the most time possible hitting the large Wraith. Again, last hit as many as possible with Siphoning Strike. Put your point in either Siphoning Strike or Spirit Fire. I have had more success with Spirit Fire myself. Smite should be just about up by now. Potion when the previous one wears off.

You may want to pop another health potion and let it run for a little bit before engaging red buff if you don't have the "correct" runes yet. Spam Spirit Fire and only use Siphoning Strike if has it's armor de-buffed by Spirit Fire.

Go kill the golems. Again, you may want to wait a bit before attacking them if your HP is low enough. Grab Wither now.

If you still have a health potion after this, you may continue in the jungle if nobody needs a gank anytime soon. The wolves will respawn by the time you walk over there. The Wraiths will respawn soon after that. Only do this if you don't need the extra items fast.

• You want to be 2 steps ahead of your enemy at all time.
• Buy wards. I cannot stress this enough.
• Break away from the pack to push, push, push.
• Farm your Q at every possible opportunity.

Do these things and win!