Sixth Term Examination Paper (STEP), 1x interview
Discussion of 10 questions set beforehand
Practice doing interview-style and STEP questions under timed conditions
Past questions, STEP classes and online workshops
'Nerd out' and talk through your thought process
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Test taken: Sixth Term Examination Paper (STEP)
Number of interviews: 1
Length of interviews: 1 hour
Online interview: No
I walked into room and was given ~30-45 minutes to answer a sheet of 10 questions. Afterwards, I went into another room with the interviewers along with my workings and discussed the questions with them.
I looked up interview questions and practiced under timed conditions. Questions from the now defunct STEP I also helped out.
Past paper questions,
Nerd out essentially, if you don't stop nerding out and talking about what works and what doesn't they'll eventually clue you in on what to do without it seeming as if you needed too much help.