If you fly in the UK in a G registered aircraft, then I recommend you use the splendid CAA CAP407 logbook. It has detailed instructions on how it should be filled in. All comments so far are addressed here.
Personally, I expect my FOs to log their PF time in the P1 column as P1S with my name as commander. When they are PNF then their time goes in the P2 column. I also suggest that when they are PF the time from takeoff to touchdown is entered in the Instrument Flying column.
For my logbook, it's all P1 however I also log the I/F time and keep a note in the remarks column as to the name of the FO and whether I was PF or PNF for each flight.