Set a rough schedule for when you'll be working, but don't get too attached. Remember, this is flexible work from home, not "I'm-a-renting-a-flat-in-the-city-to-never-leave" work from home.
Send a group chat to your colleagues, but only if you feel like it.
Use all caps to convey your seriousness.
"I WILL BE WORKING FROM HOME TODAY FROM approximately 10am TO 2pm-ish. DON'T WORRY ABOUT ME, I'LL BE FINE."
Take long walks around the neighborhood to clear your head.
Get distracted by squirrels.
That's it! With these simple steps, you'll be the most efficient WFH worker in all the land (or at least, you'll be able to convince yourself you are).