I made an account just for this, because I remember how desperate I was after finding mostly negative posts about this, so if you're in the same spot I'll try to give some hope.
For context, I've had costichondritis for roughly four months total, have been completely pain free for the past two months and completely able to workout again without any restrictions.
So, for the first few weeks, if you do workout, don't do anything at all. When the pain is almost constant, I didn't actively do anything aside from from sleeping on my back religiously and minding my posture. I also really recommend heat and gel against pain, mine was an extra strong diclofenac Gel. When the pain wasn't constant anymore, I started with a lot of stretching and mobility which helped a lot. People really love the back pod, and I'm sure it's absolutely amazing, but I personally couldn't afford it. So I strongly recommend either a peanut ball or a rolled and then folded towels, so that it's roughly the shape of a back pod. I did that morning and evening for a couple weeks and pain slowly started getting less. It takes its time, it feels hopeless, but it will go away.
Lastly, when I was itching to workout again and only had pain with specific movements, I started reintroducing all my exercises with a lowered weights. If an exercise hurts, stop doing it and try again a week later. It's better to start low and work yourself up again than to rush and flare it again.
I know it's not much and there's no secret to it, but while I was recovering, I was really worried because I didn't read much about people who did. So, take this as a sign that, if you're a little mindful, it'll very likely go away.