Once a day, think of one simple thing that a depressed person would not do. Do that. For some examples:

• Take even the shortest walk around the neighborhood.
• Read just ten pages of any book.

Also, think of something that you do especially when depressed, and avoid doing it. For some examples:

• Don't watch TV
• Don't scroll on social media

Sometimes habits become compulsive, you can do a lot for yourself by adding even a small amount of friction to doing those things. For some examples:

• When I notice that I go through periods of poor sleep due to habitually reaching for my phone, I might remove the charger from my phone and put it in a living area and charge it there at night. I might prepare 8 hours of podcasts and connect my Bluetooth before bed so I can have something to take my mind off things in case wake up or cannot sleep.
• Simply logout of social media sites, so that I'm forced to take a deliberate action to log back in to start doom scrolling again.

For more proactive maintenance things I try to do, which are important to establish when I'm mentally in a good place, I might make a weekly list of happy chores, like make sure I talk to a family member, make sure I talk to a friend, make sure I actually see someone in person, or play a video game in a way that I'm making progress at it, or spend 10 minutes practicing an instrument.

You need to exploit your good times to establish patterns of good habits for your hard times.