Ok so here it is. Get 1 x cotton air filter. You can get these really cheap from car modiying shop or e bay. Cheap and nasty works fine cos we are only trying to keep carbon in and not stop microscopic particles of dust out of an engine. Get a bag of activated carbon. Got mine from a pet shop. Turn filter upside down. Open bag of carbon cutting only one end of net bag and pour into the filter, tapping the filter as you go along to make sure it is filled properly. Fill up to neck. Some people like to crush carbon but this works for me and if it ain"t broke............. Get a piece of pvc pipe or a bend as i have. Place net bag over one end. (preferably the end that fits into the filter. Insert into filter and tighten the jubilee clip on the filter. That is it. Works gret on small grow boxes. Just clamp it to your exhaust.