A few conversations with professors followed by revision of perfomance in prior classes of related study / area. A majority of this process was demonstration of interest in teaching and having had a good track record in said department.
The process starts with sending an email to the PI. Then, an on-site presentation to meet the lab. Decision in 1 week. Ask for reference letters. Extend the offer after the reference is cleared.
Interview questions [1]
Question 1
Why this lab? What's your goal? What do you want to do next?
The interview was really nice. My future PI wanted to know why I wanted to change research fields and he also told me about all their research. I was honest and explained my interest and motivation and he mentioned that Stanford would be a great place. He mentioned that we should get funding so we should write some grants together.
Interview questions [1]
Question 1
Why do I want to change field of study after my PhD?