STEP, 2x interviews
Interview 1: Topic-specific questions; Interview 2: A-Level content questions
Use practice papers
Don't stress; keep good A level notes; believe in yourself
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: 4 hours
Length of interviews: 30-40 minutes
First interview: Got to choose one of two topics for my interview to be based on. This was nice, as I at least had an idea of what I was going to be asked. Nothing in my interviews was beyond the A Level course! I think there is a misconception that you have to read and learn loads to prepare - but they were testing the depth of my knowledge, not breadth, so they wanted to just see how well I could use the stuff I already knew.
Second interview: No topic choice. More random questions, so couldn't really prepare as much, but once again nothing that came up was beyond A level content. They guided me through some parts.
I took a practice paper every week. I took it slowly at first, completing the tests untimed, then began timing nearer to the exam to get used to the feel for timing. I marked the tests myself to pick up on recurring useful tricks. I also had to learn formulae as we didn't get formula books in the exam.
I can only speak for maths, but don't stress yourself beyond reason, as it doesn't help at all. It's easier said than done, but still worth being said.
The interviews are most definitely not worth worrying about, as they were actually quite enjoyable! Provided you've applied for a subject you like, they should be enjoyable for you too, as you'll be discussing interesting questions with the best in the field. Work hard for your A levels and keep on top of revision and notes; a good set of concise notes written throughout your time will make it much easier when it comes to revising for interview.
So, to summarise, BELIEVE in yourself, don't stress too much, and keep working hard.