when should i put the air pressure in the expansion vessel of my wor24cdi?

i've been having problems with soaring pressure on my worcester 24cdi but after reading up a bit i realised there was no air pressure in the expansion vessel and i was pressurising the system purely with cold water hence the soaring gauge when system warmed up so i know now i should drain some of the water and pump the expansion vessel with a footpump, what i need to know is when should i do this ....how much pressure should i put in? ..... should i do it with any water pressure in?..... and i presume i do it with a cold system? any help would be greatly appreciated cheers