-Your team has at least 3 AD champions that can benefit from your Armor Pen.(especially Lethality champs and high Ad base dmg champs like Urgot/Illaoi)
-Enemy team has at least 2 great targets for your W Cripple and other that are somewhat impacted by it (like Pantheon being AD, but dealing most damage with abilities anyway) or by your E armor reduction (ex. Rammus).

Try to position yourself close to your AD teammates, protect them/catch enemies with W and poke/shred armor with E. In a teamfight you sometimes can go in with your ult and benefit from magic dmg around you (it actually deals a lot of dmg).
In early game just E, whenever you have your Manaflow Band ready. If you already stacked it, you can E whenever you want and you shouldnt run out of mana unless you use your W over and over too. If you are playing backline version, you should avoid using W first, when First Strike is ready. Instead use E first, preferably on multipe enemies for more dmg and First Strike gold.

Pros
-Pretty chill to play. No insane mechanics. Mostly stress free gameplay in comparison to other champions and easy to pick up.
-Can turn a horrible full Ad team comp into a good one.
-Can shut down enemy AS based carries in teamfights.
-Can catch people out of position.
-Helps in frontline with survivability and damage from his ult.
-Has good waveclear, if you ever need to push the wave.

Cons
-No actual hard cc or engage/disengage tools. His poke is below average, when it comes to poke supports.
-Dependent on teammates. Nasus is an amplifier type of champion. His Armor shred is only going to be usefull, if your teammates actually deal dmg.
-Can't do a whole lot alone (especially, in Frozen Heart start) and in fights will use his cds and pretty much afk waiting for them to come back unless you are going all in with ult.
-Very predictable, almost no outplay potential.
-His Q is useless in this role. If it was replaced by something remotely usefull, Nasus could be a meta support, but right now he is kinda bad. Would classify him as a very low tier support. Probably not great idea to climb with Nasus support.