Complex sentence: To apply a permutation cipher a random permutation of size e is generated (the larger the value of e the more secure the cipher), the plaintext is then broken into segments of size e and the letters within that segment are permutated according to the key.
Split into simpler sentences: To apply a cipher, a random permutation of size e is generated (the larger the value of e the more secure the cipher). The plaintext is then broken into segments of size e and the letters within that segment are permuted according to the key.