The laning phase of any match can either make or break a player. While playing Nasus, you are very reliant on your Q stacks, to make farming easier against an aggresive champion like Sett, let the wave push to your side of the lane, the Tower will potentially be your best friend while you get those early stacks. Allow the tower to do enough damage to a minion, then last hit Q if you can, once you get 15-20 stacks, or feel confident enough, leave the saftey of your tower, and confront the enemy champion, but still do your best to keep the minion waves on your side of the lane.

If the enemy laner seems new enough, your best bet will be to become friends, or atleast have an early game truce with them, if they havent played against Nasus much, they may accept and let you farm in peace, to keep this trust, tell your jungler not to gank, but stay (roughly) in the area, this will both improve relations with the enemy laner, but not completly screw you over if the enemy jungler decides to try a top gank.

If the enemy laner is being too aggresive, try your best to play under tower, but most importantly, dont be ashamed to give up your tower. Giving up your tower will force enemy minions more onto your side of the map, it will be more dangerous for the enemy laner to continue pushing. If your team flames you, just ignore and mute them, cancerous teammates can ruin a game, dont let them get under your skin.

At this point, you will (hopefully) have finished your build; Kraken Slayer, Boots (Plated SteelCaps if the enemy laner is AD, Merc Treads if they are AP) and Bloodthirster. Once you get Bloodthirster, you can now solo Baron, to do this, ward the enemy jungle area closest to the Baron Pit, make sure to watch the enemy players, if you cant see the enemy Jungler or Top Laner, your best bet would be to hold off for the time being.

Once you feel comfortable trying to solo Baron, wait for the drake to be ready, once it is, have your team contest the drake for as long as possible, the enemy should be too pre-occupied with your team to notice you gone, but if they do, they will likely think you are off farming somewhere, this is the time to strike.

Begin by using E on the Baron, then Q it every chance you get. Even with only 3 items, you should still get enough shield from both Bloodthirster and Overheal, if you lose more health then you are comfortable losing, pop your Ult, it will give you 600+ more health, and a ring of constant damage. Don't be scared if the enemy takes the drake at this point, most people will recall back to base once they kill it. Recalling takes 10 seconds, and the walk to Baron will take roughly 1-2 minutes, this should give you plenty of time to finish off the Baron.

If everything has gone as expected up to this point, you will have secured a buff for your team (if they are still alive), and 300 gold for your troubles.

It is unlikely to work a second time, as the enemy team will likely be watching the next Baron that spawns, making it much more difficult to yoink it for a second time, but it isn't an impossible task.

At this point, you will hopefully have Baron buff and a full build.
The best course of action would be to split push if the enemy team is heavily defending a certain lane. If you can push into the enemy base, atleast 1 or 2 enemies will come to fight you, this will hopefully give your team the chance to push into the base.

To make sure you can secure the enemy Nexus, stand behind the Nexus and Ult once and enemy spawns, if they ignore you and go after your other teammates, use your W and slow them down, this will hopefully give your team enough time to secure the win.