I PICKED NASUS FOR THE LATE GAME
First off, Nasus is not a late game champion. "Oh but his q can stack to 1500 damage"?
Nope his Q may be infinite damage but in the end game he will simply get destroyed by CC, armor piercing and just rekt by enemy carries.

TELL YOUR TEAM THEY NEED A REAL TANK
"I went Nasus because I needed to become the teams tank"
NO!!!! Nasus is definitely not a tank. Nasus is a bruiser at best because he can farm up damage and build tanky items, but nasus has 1 crowd control ability he maxes last. This means he uses wither and he is all damage.

So first you're going to want to start off with a ruby crystal and 2 pots. Just let your opponent push hardcore it is ABSOLUTELY essential to allow your enemies minions to get towards your tower. KEEP IN MIND YOU ARE THE WEAKEST CHAMPION IN THE LEAGUE AT LVL 1. Unless you are fighting a tank that does zero damage to you, you're going to need the safety of your tower. You're going to focus on reaching 1450 gold for sheen and kindlegem regardless of everything. This is an extremely easy goal to accomplish in low elo and even in higher elo it's not too hard.

Try to get 60 stacks by the time you take your first back and teleport back to lane with a kindlegem, a sheen and a ward. You should be lvl 5 or 6 now. Trading with players should be really easy and profitable. When you are not ready to farm a minion with Q you should smack your lane opponent for damage and + 10 gold every time you have the chance, but prioritize the farm as having 300q farm at 12-15 minutes is mandatory to carry the game. If your opponent is a ranged opponent just outfarm him and heal up with your passive/q/sheen damage.

By this time you should already be outfarming your lane opponent hardcore and forcing the jungle to come to the top lane. If you are able to 1v2 then you have probably already won the game but if your team is heavy you should hurry up and get a zzrot portal and go to mid lane and drop it there. Then go back top and take as many turrets as you can as you farm your Q hard.

THE ELIXIR OF IRON IS EXTREMELY POWERFUL AT THIS POINT so make sure to get it.
DO NOT FORGET BOOTS

At this point you are destroying your bronze opposition or your silver opposition but then you find a teemo/olaf/renekton. These guys are tough and if you don't farm under your tower for a long time until you feel you can take them on you will feed. So do whatever is necessary and everything you do in game is optional so against teemo upgrade wither before spirit visage. Versus olaf and renekton you may want to buy iceborn gauntlet.

You get your team an advantage in the early game after your first back by farming hardcore until around 150 stacks where you should be trading as often as possible while still farming by your turret. You should be able to out duel your opponent easily and every stack counts. When you have around 240 stacks you should be pressuring your opponent and trading them under their tower and making sure they can't farm like most players do to the average nasus. At this point you can kill your lane opponent a few times setting them behind. Then the enemy jungle should come top and you should be able to 1v2 them both. Don't get reckless and tower dive them, as Q farm is just as valuable and also wasting 2 players time and farm is extremely effective in making the game a 5v3. Take the first tower after 300 farm and the second should fall just as easily but you will need to ward the enemy jungle in case the enemy mid laner attempts to take you on.

This may happen at some point that the other lanes fed and now you are being shut down. DO NOT LET THIS HAPPEN. Pay attention to the game and make sure you are wasting as many players time as you can by being around without being to close to the enemy. Your team should be doing something at this point, either farming/pushing/taking dragon/rift herald. Anything your team does you put them ahead. Figure out a way to put your team ahead of the enemy and give your team an advantage. No matter how many stacks you have, unless you have 900 stacks at 23 minutes and you're lvl 18 with full build, YOU CAN'T 1V5. Work with your team and you'll win.