First I´d like to say that this is my very first build ever and I´m only bronze 4. I main supp since I started playing League of Legends and even platin players told me I´m quite good at it. I hope it´s true and I hope my build for Nasus supp is at least as good as I play support. You all might think: Why the hell Nasus support???
Well I tell you why: I´d always loved to play some weird champs in support position some time because it kind of makes the game more interesting and more fun. So I started to try some rarly picked champions for example Nunu or Amumu to play them in the bottom lane and I finally tried out Nasus which worked suprisingly better than expected. Nasus, as you all know, can be a very powerfull tank with a high damage output. Therefore he is a perfect champion for toplane. He is also sometimes played in jungle which I personally find very bad because he cant get that strong and is not that effective in ganks.
Why did I think about playing Nasus as support?
-Nasus has got a very powerfull slow which can be used to help your ADC or jungler to give him more time to deal damage or finally kill your target.
-As he is a tank he has got a very high basic HP number and is therefore a strong tank for your lane at the beginning and in the end for the whole team
-By using his e you have a poke which can be very strong for the early game and quite usefull in teamfights. You can also save urself all the assists.
-Although you might think his q is useless as supp I have to say: It is not! By using your support Item you can easily stack your q to a moderate level. It will never get as strong as a toplane Nasus can stack it, but it will be still quite hurtfull especially for squishy champions like the supports, assasins and so on.

Personally I prefer to start with the e to have a poke for the beginning of the game but it is also possible to start with your q.
Start with e: Try not to steal farm from your adc but to hold the enemies back to save your adc´s farm. After you maxed your e level your q and finally your w. Don´t forget to level your ultimate.
Start with q: Combine the passiv of your supp item with the last hit with q to get 3 stacks for every minion you kill! You and your mate will get money, heal and you will get your 3 stacks. Do not farm without a stack of your support item! Max your q and after this your poke. Finally you max the w but ont forget your ultimate.

If you play Nasus as supp your e gets a new importance for you because it is very important to poke the enemies down. You have to keep the enemies away. Therefore you should always focus the adc because he will try to get the farm. Combine your e with your w to deal as much damage as possible. In fights with your lane opponent or in teamfights you should always focus the one dealing the most damage to your team. Try to place the e under him so he is right in the middle of it. Use your w to slow him down and try to use the slowing effects of Iceborn Gauntlet/Frozen Mallet on him. The longer he and his teammates will stay on your Spirit Fire the more damage you will deal to them. In teamfights you can use your ultimate to steal hp from them to maximize the damage dealt to them.
Never the less you should try to stack your q by combining the last hits with the passive of Face of the Mountain. The more stacks you get the more dangerous you will become for the enemy team! Try to focus the big minions to get as much money as possible!