i'm not exactly pro nasus, but he is what i win by far the most with to the point where i call him my free elo pick.
i max E>W>Q and my first 3 points go into Q>E>E. used to add W, but you don't have killing potential at level 3 so it's only a decent escape tool and you will level it at 4 anyways. E and promote make you a very hard pusher to deal with. i find that's what makes nasus viable in dominion despite the inherently weaker late game Q.
maxing CDR and building tanky is what, for me, makes him incredibly powerful. you push harder, you can chain withers, and your Q farm is still reasonable even though you max it last because of your CDR. to put it into perspective, you have 4 second cooldown on maxed Q. with max CDR, it's 5.2 at level 1. good games lead to me defending 3v1 and killing people at the tower when i finish frozen heart, fon and shurelias faster than normal, and the average game i can hold 2v1 vs ganks and even hold their bot tower consistently if the lane match up favors nasus too much.
my core early game items are merc treads, glacial shroud, kindlegem and a negatron if i'm vs an AP lane. my first buys are a chain vest, mana pendent, boots, 3 reds and 1 blue pot and my goal within the first few minutes is to push the lane and get a glacial shroud, then mercs, then kindle.
i might as well throw it out there, i stole this from a guide that suggested it's possible to get a glacial shroud at the beginning of the game by getting chain vest and mana crystal with 1 pot, capping your tower, recalling and using the extra money to buy glacial. i don't suggest this because 2 bottom is an auto cap on your tower and you will not enjoy having only bought 1 pot if you are 2 bottom and can't recall.
then i build to counter whatever does dmg to me, looking to get a FoN, finish frozen heart, finish shurelias. what to get next is not set in stone, it usually varies between thornmail, aegis and odin's veil. basically you either still need to get more defense against someone well fed or you go aegis to be all around stronger. aegis is the strongest fifth item buy imo but sometimes you need to overcompensate. for the same reasons, you finish one item or the other based on threats. if you're retardedly fed for some reason and can afford thornmail and odins you basically go into godmode vs everyone. i've never gotten six item nasus but the theorycraft says he'd have like 9k effective HP with FoN regenning him so i'm not sure how to even die without clicking hold position for 10-15 seconds.
a question i haven't quite been able to answer is how to buy for an AP lane, because i just haven't gotten enough games where there is an AP champ threatening enough to warrant early MR like cass. just haven't gone up against one enough times yet. my common sense tells me to start with the cloak or even negatron, but i don't really have experience to say anything for sure.
tl;dr i max CDR then just build tanky, E>W>Q, afk push with E+promote and feed on poor lane matchs ups to turn into an immortal bot lane monster