CSAT test; 2x interviews
First interview: personal statement in first half, new concept and logic in second half; Second interview: maths questions
CSAT past papers; maths preparation
Relax!; socialise with other applicants
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.
Number of interviews: 2
Skype interview: No
Time between interviews: 30 minutes
Length of first interview: 30 minutes; Length of second interview: 30 minutes
In the first half of my first interview (Computer Science) they asked me questions about my personal statement and the second half involved introducing me to a concept from Computer Science, followed up with a series of logic questions building on top of each other about it.
My second interview (Maths) just involved several maths questions I had to solve.
Before my interviews
I didn't really have any help from my school or family as I was applying abroad from a school that doesn't really have people applying to the UK, so
My advice for the interview would be to talk and relax with the other applicants while waiting for your interview. Last minute