I always max first q, Siphoning Strike is your best weapon against your enemy. If you have a good start with farming in your game as Nasus, most probably it will end sooner if your teammates allows it.

Next I maximize is W, Wither helps you chase down opponents. It also gives you a 85% chance of kill if you caught the enemy alone.

Lastly, maximizing E, Spirit Fire. Spirirt Fire gives out a small damage to enemies at start and it can be used for farming.

Fury of the Sands(R) helps you survive clashes.

Your passive skill is Soul Eater, at early game you don't really need a Doran's Blade cause you have your very own life steal.

Last Hitting Creeps with Q helps you through your game.
When you have killed a Creeps, Monsters or Champions with Q, it gains additional damage.

THE MORE YOU KILL WITH Q, THE MORE STRONGER YOU GO.

Pros
-Gains more damage with Siphoning Strike
-Can Hunt enemy

Cons
-Mana drains easily when E is frequently used
-Ability depender
-Not really that Tanky Type

Thank you for viewing this Guide hope it helps you a lot.
By the way this is my First Guide sorry if you don't like the build ahahaha
GOODLUCK with your games!