Put racks inside bag.
Pour in half the bottle of 'stuff' (if you are a slattern like me you may need the whole bottle).
Seal the bag, squeezing out as much air as possible.
Turn bag over and round so 'stuff' coats it all.
Stand bag up, fold over top.
Tilt by 90 degrees so on side and fold other side, trying to keep 'stuff' in main part, not folded bits.
Lie bag down on draining board NOT something you don't want to wreck.
Leave for few hours, overnight is fine if particularly minging but turn over part way through.
Open bag WEARING GLOVES.
Remove racks, rinse vvvv carefully, outside under garden tap/with hose is easiest.
Wipe down each bar to make sure all 'stuff' removed
Leave to dry
Admire sparkling clean racks.
(I think there are more detailed instructions on the actual bag - the box isn't very clear).