As taught to me by my grandfather when I was a kid. Although he wasn't a Cockney, more a fan of word puzzles. This isn't rhyming slang. You've got to pick the letter each clue goes with. Say to your self... 'A' for ___, 'B' for ____, etc. I'll give you a couple of examples:
N for Lope (envelope) / B for My time (before my time)
(Some traditional ones are a bit difficult or old fashioned or rude, so I've tried to pick easier ones, though purists may not like them).
I think I've put the hardest / most unconvincing ones nearer the end.
Evil aliens have decided to kill everyone on Earth! Fortunately, they've given you two minutes to save as many as you can. All you have to do is name a country to save everyone in it.