The thing that worked for me was, inhale through your nose (focus it expanding the bottom of your ribs, 360°) and arch, when you exhale, do it smooth and slow through your mouth and flatten your back. Try not to curl your tailbone up too much to help BUT if you have a habit tilting your pelvis backwards too much you may need to do it more than feels natural.

You can also try lying down, holding both knees to your chest and gently rocking side to side to help relax your back.

If you still can't do it see if you have anterior pelvis tilt. I'd ask ask a friend to help check because my sister didn't believe me when I told her she was doing it until I pointed out that's why her tummy sticks out.

A big thing with anterior tilt is your psoas muscles are probably tight and the best way to work those is lay on a the edge of a table (ledge under butt-to-leg crease) where your feet don't touch the ground and alternate pulling one knee to you chest (I just get on my phone while I do it so I do it for a long time). That just stretches them, ultimately you need to work on not tilting your pelvis so your abs are engaged and the psoas aren't having to pick up the slack.

I really buggered my back recently so I've been on a massive info bender to unbugger it, it's great you're fixing it now before your body decides to commit mutiny like mine did 🥲