The decryption process for the Rail Fence Cipher involves reconstructing the diagonal grid used to encrypt the message.
We start writing the message, but leaving a dash in place of the spaces yet to be occupied. Gradually, you can replace all the dashes with the corresponding letters, and read off the plaintext from the table.
We start by making a grid with as many rows as the key is, and as many columns as the length of the ciphertext. We then place the first letter in the top left square, and dashes diagonally downwards where the letters will be.
When we get back to the top row, we place the next letter in the ciphertext. Continue like this across the row, and start the next row when you reach the end.