a situation where someone is trying to show someone else how to do something which they do not know how to do themselves.

I tried to explain how the software works, but it was a case of the blind leading the blind, really.

• the cream of the crop
• the curtain comes down on
• the curtain comes down on something
• the curtain falls
• the curtain falls on
• the curtain falls on something
• the cut and thrust of
• the cut and thrust of something
• the darkest hour is just before the dawn.
• the day of reckoning
• the dead hand of
• the dead hand of something
• the death knell
• the demon drink
• the devil finds work for idle hands.
• the devil take the hindmost
• the dice are loaded against
• the dice are loaded against someone
• the die is cast
• the dog days
• the done thing
• the dreaded lurgy
• the dust settles
• the early bird catches the worm.
• the earth moved