Hi
Both Len and martin are spot on.
You need a dicta-phone and basically as soon as you go into the property you start talking to yourself on tape.
'' front door is white in colour but is marked in the top left corner, it opens fine. Inner door is off white in colour and is cracked down side with 2 nails acting as coat hooks etc etc etc
You have to note every stain, mark, damage (no matter how minor) if it needs cleaning, do they need an electrician, plumber, decorator etc, etc
meter readings will also be required. Best thing is to buy the dicta-phone and practice on you own place beforehand.
As also stated you will charge for this as you also have to type up what you have written in a format of some kind for the letting agent. There are companies that do this full time.
Good luck!
Regards
Martin