I went to Meetups and talked to strangers. I want to say I had a specific practice routine, but the fact is I just went out and did it.
At first it was awkward and tiring, but because I truly wanted to get better at being a conversationalist, I kept doing it. I picked up a few conversation starters like "what keeps you busy these days?" but a lot of it is just reading the person and coming up with the right questions. And there were definitely ups and downs but now I'm able to walk into a room and have an easy conversation with a stranger (most of the time).
It does help that I can read social cues very well, an ability without which would have made it much harder. But we are all dealt a different deck of cards, and the idea of growth mindset is not to achieve some specific goal (we all have limits), but knowing that it's possible to get a lot better from where we started. That change of mindset is huge -- I've often been surprised by how far I'm able to push myself by not allowing myself to be put in boxes.
Introversion is not a binary setting. It's a social energy tank. And I've discovered the size of that tank can grow with practice. I've also discovered that introversion and antisocial tendencies are orthogonal concepts. Many introverts who have learned to grow their tank can be very social people. Their tanks drain much slower than unpracticed introverts.