As already stated to be a FI you must be pilot in command
AMC1 FCL.050 Recording of flight time
(b) Logging of time:
(1) PIC flight time:
(iii) the holder of an instructor certificate may log as PIC all flight time
during which he or she acts as an instructor in an aircraft;
How you log instruction time is up to you, there are no specific requirements. Personally, I log the students name and the Exercise number. I also make my own column in my log book to record it.