"buy 2L of unscented, plain bleach. Homebrand is fine.
buy 2L of white vinegar. Again, homebrand is fine, but it must be white vinegar, not malt or wine or anything else.
In the bottom of the fermenter put 1.9L of the bleach. This amount is for a 30L fermenter, if yours is different size, its 62.5ml per Litre. Put in all your other brewing equipment.
fill the fermenter almost to the top water, just regular tap temperature.
add the same amount of vinegar as the bleach, so 1.9L for a 30L fermenter. Stand back a little when you do this, definately do not put your head over it....it will start to produce chlorine gas. Dont breath it in. NEVER add vinegar (or any other acid) directly to undiluted bleach (or any other alkali.) always add the alkali to the water, then add the acid.
top up with water, and put the lid on. make sure the inside of the lid is in contact with the water. (if it was full, it should be.)
allow this to stand for 20 minutes. Open the tap and allow it to drain out. Rinse well with water.
refill the fermenter to the top with water, allow it to stand for another 5 minutes, then drain. This is really just to make sure on the rinse, if its rinsed really really well, you could get away with not doing it. But I would.