I'm a big believer in someone naturally pushing themselves (progressively) based on their belief in the TMS diagnosis. So that was what was first, I built the evidence up. Once I was confident enough that pain didn't mean damage and I wasn't going to further injure myself, then I challenged the conditioned response.
What causes a conditioned response? Fear (and by extension attention) and avoidance. So I had to start sitting and stop avoiding it (because avoiding something confirms it's dangerous), but I also had to catch and challenge my anxiety both before and after the activity.
So before I had to sit, the fear started swelling up and the dread started kicking in, in an attempt to stop me from sitting (which is normal). I would challenge it in the moment and say "There's nothing wrong with my body, I know this from the evidence, this is just a conditioned response. The sitting itself isn't actually causing the pain but rather it's the belief that the sitting is dangerous that is". That would calm me down and it's challenging that danger response to the activity which is fuelling the conditioned response. I would say the same thing during the sitting when in pain.
I think equally important is what happens afterwards. Before I knew about TMS, if I hurt more after sitting, then there would be tremendous guilt and fear, as if to say "why did you sit on that chair, look what you've done and now we are going to be in pain for a week". This only confirms to myself that I was wrong to sit, and therefore it must be dangerous. After I knew about TMS, if it hurt after sitting I would say to myself "Don't you dare blame it on the sitting and regret it, it's just a conditioned response and you were absolutely right to sit as there's nothing wrong with you, the only way to break the conditioned response is to sit without fear".
I did that enough times until my indifference to sitting built up - it hurt but I didn't care (or at least not as much as I used to). Because the main fuel for the conditioned response is fear, I was cutting this off and the conditioned response started to break up (it didn't disappear overnight). Once it started to break up, even more things that wouldn't make sense if it were structural started to happen, like one day it hurt more on a comfy couch than on a wooden stool - that only increased my confidence and improved my reactions. I began to be able to link my symptoms more to my emotions as the conditioned response was no longer distracting me (as it was breaking up) - so for example when I was more stressed I couldn't sit for 10 minutes vs when I wasn't as stressed I could sit for 40 minutes with a lot less pain. I knew at that stage. That's how I broke it, and even though I can still get minor symptoms from stress (which don't impact me), nothing physical (including sitting) triggers my pain at all - I can sit for 10 hours and have no change, whereas I used to not be able to sit for 10 minutes without it turning into agony. It's still hard to believe sometimes and feels like magic (because I haven't change anything physically), but conditioned responses can be broken no matter how real they seem