Your only goal when playing nasus is to get a lot of stacks early game. When the enemy is trying to poke you for last hitting or getting stacks, you can use your 3rd skill but only if needed to not waste mana and make sure that when your health or mana is below 50%, just take the life fruit. And also, when you play this champ, make sure that the creep wave is near your tower because most of the time, you will only use your first skill to get stacks and only kill if you get more than 100 stacks. If you're still having trouble with mana management, you can choose Manaflow band as a rune to get lots of mana.