The general idea is to create a PRNG using the cryptographic hash. Since these hashes generally produce far more bits than needed for scalars (64 bits), you can

The general idea is to create a PRNG using the cryptographic hash. Since these hashes generally produce far more bits than needed for scalars (64 bits), you can mix the extra bits for the next cycle of the PRNG and emit (say) 64 bits per cycle. The sequence of the PRNG output is the 'pad'.