So I hate laning against Garens, because he does not need to leave lane. No mana to manage.
If you are a Garen main and you lane verse a Nasus I would first look at what CDR the Nasus is running.
If he is running scaling CDR he can stay in lane if he wants. He does not need to back to get more CDR items and can stay in lane with you. I would roam if you think he is a seasoned Nasus. You can either look him up while the game is loading or you can look at his starting CDR. Typically low percentages indicate scaling CDR.
If he runs flat CDR then I would stay in lane. Flat CDR will likely be higher between 10%-25%. Nasus players that run flat CDR need to back to get more CDR items in order to stack efficiently. Garen can stay in lane as long as he wants and Nasus will miss a wave if he backs and doesn't have TP.
I would stay in lane early if he is running flat CDR, just wait until he runs out of mana. Then use your TP to make a play after he TPs back to lane with his first back. If he stays and has low mana call for a dive.
I run flat CDR because I think it helps more with early stacks, but against manaless champs this is problematic for me because I need to back at least twice to get CDR items. Which means i need to burn my first TP once to return to lane.
Also another tip only last hit minions and rarely spin to shove. If you only last hit, the Nasus may shove the wave with autos. I always try to auto attack to life steal, and at level 1 and 2 I try to get the first several stacks before I start taking a lot of harassment. But if the player is really smart they will LET this happen and the wave will shove to them. Most Nasus players don't like crossing the river. If he does call for a gank.
Wave management is really important with Nasus and typically silver and gold players try to harass Nasus and this shoves the wave to his tower.
If you ONLY last hit against the Nasus, he will probably accidentally shove the wave. Really good Jax players manage the wave way better than me and literally freeze it by their tower.
I can't farm or roam and I have to take my junglers camps if I want stacks while I wait for it to shove. I am probably not strong enough to make any plays.
I would say wave management against a Nasus is MORE important than early harassment if he is a pretty decent Nasus.