For Nasus, killing minions with his Q(Siphoning Strike) is very important as this adds damage to the ability, going up in 3's for normal minion and 6 for anything bigger(canon minions, super minions, champions and large monsters) This means that Nasus is a late game champion who is very weak in the early game. Try and play safely and just freeze lane by only Q ing minions. Do not try and trade by using e as the depletes mana very quickly and although it damages your opponent and the minions is is better to conserve mana to stay in lane longer to farm q stacks.

This build is strong as it makes Nasus very tanky while still incorporating CDR, Armour, MR, MS and damage. Triforce(Trinity Force) is a must have for Nasus. Boots can be swapped out for something else however I prefer lucidity as it increases CDR for more Q action. Dead Mans Plate is good for Increased MS, HP and initial Damage. Spirit Visage is useful for the increased passive healing so that Nasus is more sustainable as well as HP and MR. Frozen Heart is also effective as it provides mana, CDR, Armour and slows down AA from Opponents. And last Essence Reaver i find it nice to have this as it provides Crit Chance, Damage and a mana sustain (on crit) however this is not essential and can be swapped for another tank item if necessary.

Nasus is very simple in Early game you just focus on getting your Q stacks, middle game some prefer to continue farming Q stacks or to use the Q stacks already farmed to help team by pushing mid. If you wish to continue farming make sure your team does alright without you, this is strong as when you are in a team fight you do insane amounts of damage. For the second path of then assisting your team make sure u still get stacks on minions and monsters and champs but it is not your primary focus. Late game you should have minimum 400 stacks (+30 min game) to one shot (or two) the ADC and mid laner. It is also very strong for pushing towers as Q stacks and damage apply to towers as well and as a high HP tank u can last under tower for a long time.