This build is only for those who play like the guide suggests you to. If you don't you may fall off track or be no help.

Your runes always have to be your AD or tank ones. Giving him AP runes don't really benefit him a lot as an tank whose only AP move is E.

Build tank overrall. Unless you're doing some troll building.

My prefer builds:

Frozen Heart-Mana, armor, cd, and its passive
Mercury's treads-Magic Resist and debuff resist
Spirit Visage-Your heals are stronger due to passive and HP and magic resist
Dead man's Plate-Passive and HP and armor
Trinity Force: It's a good hybrid item. Worth buying.

Situational last items:

Guardian's Angel: If everything is ok.

Warmog's armor if you need more heal after fights

Randuin's Omen if there's are fed adc or yasuo.

Thornmail if they have heavy AD. But barely any Ap

Locket of the Iron Solari if you're in team fights a lot

Banshee's Veil if you're in 1v1 fights once in a while.

Hextech Gunblade if you have a lot of stacks

Sterak's Gage: Another choice if you don't want Guardian's Angel'

Ohmwreker if you're diving a lot

Sunfire Cape if you're in team fights a lot

Well depends.
There're four ways:
1. WERQ
2. EWRQ
3. REWQ
4. RWEQ

You don't have to always ult to kill.

Q-Deals damage and for every stack he has, his Q does more damage. Fighting against a decent nasus in a hour game fight is probably one of the dumbest thing you can do unless you're really ahead. (Single)

W-Slows enemy and lowers atk spd of theirs(Single)

E-Creates a zone that does AP damage and drains some HP from them and does a smart damage overtime as they stay in the zone. Also weakens armor, the amount varies on what rank the skill is on.(AoE0

R-Buffs yourself to use enemies' max HP to attack them back. You also gain a huge burst in your combat stats.(Buff)

This build is my prefer build for a balanced game. If something goes out of balance in the enemy team. For example, Vayne gets fed. Build more HP and armor items. It all depends on your team and enemy and how the game is progressing.

You won't see him much in ranked. Due to the fact that he has many counters and many ways to get countered by the ones he counter himself.

Nasus has really weak base attack damage. Don't build AD items, it won't do much.
Nasus is really easy to be ganked due to him having no moves to dash and move around. Making him vulnerable to gank when he's pushing.

Not a good jungler. You should really ignore the enemy mostly unless you're getting ganked and you're low. Other wise just farm. And always ward bushes. Pink wards don't help too much but they're quite helpful at times. For example, if your pink ward area suddenly goes black, you know someone's destroyed it or near it.

Gank for your team when you want. Help their team fights when you have 201-300 stacks. If you trust them to win. Start helping at 360-540 stacks, or else your team when start dying or losing in fights.

Last hit minions with Q. Super minions and siege minions and large monsters in jungle give you 6 stacks. Little ones give you 3 stacks. Last hitting champs give 6 stacks too. Focus on farm instead of ganking a lot for your team. Let them gank if they want, cause it's better than no assist or kills for you. I personally don't see ganks for me a lot as Nasus, my team usually wants me to focus farm.