This is the build I use for Nasus on bottom lane. It's pretty much easy mode, all you have to do is spam Spirit Fire, its large enough to hit creeps and your opponent. It's also easy to get your Siphoning Strike stacked up. For a more in depth guide go to firerebels guide here. It's where I got the base build for my own build.

For boots, just play it by ear. Any of the 3 I listed are good choices.

After purchasing Trinity Force just stack defense items according to your needs.

The speed boost you get from Summoners Resolve is soo good it makes is pointless to get Ghost. Revive is used in case you die and need to get back to bot tower and defend it asap.

Your gonna be able to harass your opponent down and if they get to half health and you Q,W,E,Q and auto attack you will get the kill. Ignite is just there in case they're quicker.

This is tough to take down early game and helps a lot mid-late game. I would recommend trying it.

Nasus owns bot lane. A good Heimerdinger is one of the only champions I've had problems with. Promote works good early game against Heimer. Also harassing him and getting help from a team mate is a good way to go. That way the two cap on his tower is quick and you can Recall back to gear up.