Undo one of the hoses from the the bottom of the header tank, drain water, Undo the bottom rad hose, then get a hose pipe and poke it in the end of the hose you took of the bottom of the header tank, and run water through till it becomes clear. Refit all hoses fill system back up with cooant/water. Make sure when filling u fill up as much as poss before starting the engine, then start the engine put ur heater on full wack and top the header tank up accordingly