Wait for seeds to ripen fully on the plant before cutting off the flower spike. Remove in several pieces if it's as large as this.
Have a paper bag to hand for transporting the flower spike as seeds will quickly fall and be lost if you carry them far.
Gently tip the bag out onto a sheet of paper, making sure you remove all the seeds. Pick out any bugs or pieces of debris.
Funnel the seeds into paper envelopes, not plastic bags. Write full details of the plant on the envelope, including the date.