Using a seed tray, sow Allium seeds from January to mid March at 15-20C (60-68F), on the surface of a good quality seed compost such as John Innes 'Seed and Cutting', then gently firm down.
Keep the soil damp but not wet, and place the seed tray either in a propagator or seal the seed tray inside a polythene bag until after germination - which usually takes about 3 months.
Make sure that the tray is placed in a bright position but out of direct sunlight.
Giant ornamental onion seed needs a cold period in order to help break any seed dormancy before they can germinate and so if germination does not occur by the end of 3 months, transfer the container to a fridge (not freezer) at 5C (40F) for a further 3 months.
Check regularly while in the fridge and remove once seeds start to germinate.