Step 1: Compare each letter of XEROX with its code YFSPY. X to Y, E to F, R to S, O to P, X to Y: every letter moves forward by one position in the alphabet.
Step 2: Apply the same plus one shift to PAPER. P to Q, A to B, P to Q, E to F, R to S.
Step 3: Combine the shifted letters to get QBQFS.