This build focus on survivability and having high damadge on your Siphoning Strike. The most usual lane you see an nasus on is top if you go on bot or anything you are stupid it's hard to stack Siphoning Strike and get any farm when you are against an ad carry with an support. The biggest issue for nasus is that he hasn't an strong laning phase early game he need the bonus dmg from sheen or trinity force but when he got both of thoose he is really strong. If you get early kills with nasus you will have alot higher dmg than the other top because during the time he is dead you can stack Siphoning Strike.

Now the runes:
Armor pen Mark - It's good for farming and the most of the champions that goes top have armor runes.
Armor Seal - It's good for sustain in lane and being even more tanky and most of the chamions at top are ad and even if there are some ap they can't harass much with their basic attacks.
Magic Resist Glyph - Even if it is more usual with ad tops i good to have magic resist cause your magic resist items are often bougth mid or late game and you will need some magic resist until then. And it can be some aps top too.
Armor Quintessence - You need the extra armor.

You need tanky Masteries if you want sustain you can play more dmg on nasus but it's often better with a really tanky nasus. You need the health, armor and magic resist, the cooldown is also important for stacking your Siphoning Strike. Often that extra armor will help you win the lane.

First the cloth armor is a good choice cause you need the armor and you can have 2 additional hp pots that will be helpful if you are focusing on stacking you Siphoning Strike and not taking kills. The philospher's stone is useful for the health reg. and mana reg. you will notice when you are using Siphoning Strike that it cost some mana when you are farming it really much that's why you need some mana reg. you won't have it later you will instead sell it when you buy bloodthirster. The heart of gold is good for the extra health and the gold. You can have lower farm than your enemy and not as many kills or assist and still be ahead with gold. This is really important if you are meeting an ranged who can harass really good and still farm. The sheen is really important i have seen many that plays nasus that starts with phage for the health, dmg and slow but sheen can give you instead of 250 dmg with your Siphoning Strike 400 dmg and if you stack really good you can have trinity force and 500 dmg stacked that will give about 1k hits on everyone that has a low armor. That's why you choose trinity force. After trinity force you have to buy a warmog you can buy the frozen heart before warmog depending on aps or ads are fed. If ads is fed you want the frozen heart early but if aps are fed you need health so they can't burst you fast enough if no one is fed you still go for warmog cause it's good against ad and ap. Some may say thornmail on nasus but that is only good when you are against an full ad team not when you are against an normal team because you will need the frozen heart for Cooldown, mana and armor. After you bought both of thoose you need force of nature this can be changed to an thornmail if you are against heavy ad team. And if you aren't go for the force of nature it gives alot of magic resist and hp reg. your last item will be bloodthirster you will need the extra dmg and the lifesteal you have with you passive will be 40% if the bloodthirster is full. That will let you kill their ad carry easy and you will almost get up all hp you lose (if you aren't stunned or against 5 champions alone).

Here is some useful information about your Siphoning Strike it is really good with armor pen. and trinity force. You can stack it on teemo mushroom and wards so and oracle can be bought if you know they are warding alot. There isn't an cap so you can take towers in 1-hit champions too.

I use flash and ignite when i play nasus because it's always good when it's 2 on 1 at top lane if they turret dive you can ignite them and slow them and most of the time get a kill, ignite are also if you are an aggresive laner that goes for kills and poke them out with the stick. You can pick heal for a bit more health also good when they are diving you cause you can heal and use ulti. If you don't like the spells heal ignite you can use exhaust to reduce their dmg and as an extra slow. You can use teleport as an escape or if you wanna go backdoors you can place and ward at their tower and then you can teleport to it and pick up an tower and just buy new wards and do it all the game and hopefully you will win on push if you use it as an escape remember to be careful because stuns will remove your teleport and stop you from escaping. If you don't like flash you should use ghost because if you don't have any escape options you will be easily ganked by their jungle, flash is also a really effective way to get to their ad carry you can just flash and W, E, Q and kill him easy. Smite is an option for nasus but it's not the best you can play nasus jungle but he dosen't have good ganks or that fast jungling, but he can stack his Q and his lifesteal are also good. There is more spells but they aren't good to nasus so if you can try to pick one of these. And if you are jungle nasus you may have to do some changes in the build you should have smite, exhaust in jungle for good ganks.