Nasus and other champions that stack such as Veigar and Sion are very much based on farming as well as kills. Farming is a very important factor and if you want to win/carry games you will need to practice farming on Nasus which may be hard at first but will become easier as you go. You're doing ok if you have 100 stacks at 10 minutes. That does not mean you should have 200 at 20 because you should have CDR and be at around 275-300 stacks. Try and get as close to 500 stacks at 30 and feel free to go farm the Jungle.

A very important thing to do on Nasus is freezing lane. This is so you can freely farm without having to do much and having their whole team gank you. Most people know how to freeze lane, but ice born gauntlet makes it slightly harder. Practice freezing lane by only last hitting minions in lane.

Also while farming there is no way early game you will be able to stack every minion but that does not mean that you should not get the farm.

So being completely honest I do not roam much on Nasus if my team is doing really well. Many people will say you shouldn't roam until a fairly decent time in the game but this depends on how your team is going. Nasus can push very fast so if your laner decides they will roam it is an opportunity to take their tower. One of the things I have done is pushed my tower to the inhib tower and then put a banner of command minion top and then push bot towers.

Of course if you are in lane and your jungler gets in a fight in the river you should go help. Also if you need your team to get an advantage so you can farm help out in a couple team fights.

If Nasus is doing well normally he gets ganked very often. If multiple people gank you try and target the one doing most damage and especially target the adc (unless they are super underfed. Don't waste your time trying to kill the super tank while the adc is there killing you.

The Whole Idea of Nasus doing damage is from his Q, which means to must fight with it or you pretty much do no damage. When you are fighting someone always try and fight them under your e as it shreds armour. In lane once you get a decent amount of stacks e then q someone and they will normally be afraid to engage again. Feel free to keep farming and when you want a kill just W, and fight them under your e.