It's not a simple substitution like that way, so please don't make the mistake of thinking that a=U, r=R, e=U, no, not that game.

It's like, you need a starting point.
Look closely at the two cards and what features each have on the front and the back, and these features will be used to uniquely determine the placement.
However, with an unknown placement, how can we determine which substitute table to use for decoding? This requires a default value.
That is our starting point.

Simplicity is beauty, and by decoding the ciphertext message from this simplest starting point, we can find out from it how the cards were chosen and placed, and how to use this 1-to-4 substitution table.

Then from the 12 possible ways of placement, we choose that combination we need to decode the rest of the cipher message, and since you already know the plaintext, it's easy to verify that the decoding is correct, and that's when you need a little bit of patience, and a little bit of luck.