Start with the clue that has the letter "M" at the bottom of it, and use the corresponding number to see what letter has that number on the decoder. Then continue to go in order using the letters "N", "O", "P" on the bottom of each clue and match their corresponding numbers to the letter in the decoder.
For example, if we started with the letter "A", then we would look for the clue that has the letter "A" on it. If the clue letter/number combination is "A,6" then using the decoder, that means the first letter in the name of the person who stole the cup is the letter "F," since "F" has the number 6 below it. Then I would look for the clue that had the letter "B" and do the same thing until I spell out a name!