Nasus support is a very fun but EXTREMELY situational pick. I personally exclusively play this when the enemy support is an enchanter and there are no APCs on either team. This pick will almost always win lane if you ward well and play around vision, and Nasus is very difficult to pick off with his W and R. Level 1 invade with W is also strong.

Bushes are your friend. Play from inside bushes and wither when the enemy gets close. Get sweeper and a pink ward first back. You win most matchups level 5 or 7 (whichever level your W hits rank 3 at, depending on if you level Q), so try to fight when that happens. Play extra safe before then, unless you have an Ashe or Draven or Lucian, in which case you can play more forward and punish enemies for making mistakes. Your main combo is W E ( not a combo, I know) and then run them down with Approach Velocity. When you level Q, use it to execute cannons and start stacking. Your ADC won't miss 4 caster minions either. Roaming is optional. You are much stronger in lane with your ADC than anywhere else on the map. Post laning-phase, you are more free to leave your ADC. They probably have 2-4 kills and are up 20cs if you won lane. If you are both down, look for isolated picks. Not even Naafiri can escape a 7 second cooldown Nasus W. In teamfights, W the enemy ADC and E the members with the highest armor. Ult and run around slowing everyone. Rylai's and Cosmic Drive have insane synergy with your Ultimate, sometimes I build Zeke's if I'm super ahead just for fun.

If you're going AP, you need to play less agressive in lane. Poke the enemies with E when manaflow is up and play from bushes (again). I would recommend leveling Q sooner , maybe 3 or 4, to start stacking faster. In fights, your main damage will be R and you'll want to be Q'ing for sustain the whole time. My favorite part of AP Nasus is the waveclear. Once you have 2 items, you will oneshot entire waves with a single E cast.

Do not pick Nasus if:
You have an APC
Enemy is an APC
Melee/Engage enemy Support
Enemy Jhin
No autoattackers on enemy team
Your whole team is AP (idk why this would ever happen)

Pick Nasus if:
You have an ADC with hard cc
Enemy support is an enchanter
Many autoattackers on the enemy team
Your team will benefit from your E armor shred