Removal of white fly

hi please excuse me but im very new to vegetable gardening, im having a go at growing a few veg in my back garden & yesterday a saw my cabbage & broccoli both have white fly/cabbage fly, can anyone tell me how i can get rid ?
I 've read on web that a weak solution of Jeyes Fluid does the job but whats a weak solution ? knowing the ratio would be nice, i also read that bonfire ash stops the larvae coming up from the roots of the plant & also putting a collar round the base of the plant,
any other suggestions ? and does anyone know the ratio for Jeyes Fluid ?

thanks for your time =)