recharging my worcester bosch 24si boiler?

i need to recharge my boiler, but i have not had to do it before now and i am not quite sure about the procedure.When the boiler was fitted the plumber showed me what to do but that was back in april and he is not working today.I have an instruction manual but it does not cover this.
worcester bosch greenstar 25si ,wall hung rsf gas fired condensing combi boiler.