The letters are first converted into numbers
The next part of the clue (2) tells you to cut the first line in half.
The third part of the clue (3) tells you to combine the upper and lower numbers that are directly above/below each other into two-digit coordinates for the letters in the Polybius Square. The first number on the top is 3 and the first number on the bottom is 3, giving you 33. The second number on the top is 5 and the second number on the bottom is 4, giving you 54. When you get these new pairs, they will be converted back into letters:
Therefore the answer is: NUMBER TEN DOWNING ST