To encrypt plaintext using a clock cipher, use a colon (:) to separate each number (which corresponds to every letter of your plaintext). Then, every word will be separated using two zeros as in denoting o'clock (00).
Example:
Plaintext: MEET ME IN AN HOUR
Ciphertext: 12:4:4:19:00:12:4:00:8:13:00:AM:13:00:7:14:20:17
19:17:24:00:8:19:00:AM:13:3:00:4:13:9:14:24