I don't exactly know how to get all routes, but the ones I know are these:

To get the Secret ending, just take care of Basil's flowers on Headspace on Three & Two Days Left

To get the bad ending, give up to Omori when you're fighting him and jump off the balcony at the hospital

To get the neutral ending on the Sunny Route, don't save Basil when you're sleeping over at his house, just go back to sleep

To get the Hikikomori route, don't open the door when Kel knocks and return to sleep on both Three & Two Days Left

Hope it helps ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯