Assalamalaiykum.
I have a few questions about combining prayers which I hope brothers and sisters might be able to answer for me.

1. If combining Zuhr and Asr when does one pray - At the time of Zuhr or Asr?
Also if combining Maghrib and Isha at which time should it be?

2. Is it permissible to combine prayers when one is ill and how ill do they have to be? If it is a cold and fever is it permissible?

3. What is the procedure and niyah of combining prayer. Do you just pray as you would normally just one after the other or is there a specific way?

4. I may be having general anaesthetic and the effects of that last a few days. Is it permissible to combine prayers in these circumstances?

Jazakallah khair.