Fill a seed tray with seed-starting mix, and gently firm with a board or second tray to eliminate air pockets. Using a watering can with a fine spray, water the soil lightly and allow to drain before sowing.
Distribute the seeds evenly over the surface of the moist soil by scattering them from their packet or from your hand. Sow the seeds thinly to prevent overcrowding once they have germinated.
Lightly water the soil using a watering can with a fine spray, or stand the tray in water until the surface darkens, then allow to drain. Label the tray to help you keep track of what you have sown.
Provide warm, humid conditions by placing the tray in a propagator or covering it with glass or plastic. As soon as seedlings emerge, remove the cover, but protect them from strong sunlight. Turn trays on windowsills regularly.
When the first seed leaves are fully developed, prick the seedlings out. Fill a module tray with moist potting mix. Water the seedlings and, holding each by a leaf, loosen the soil with a pencil or dibber and tease its roots free.
Dibble a hole in every module and carefully lower a seedling into each, using the dibber to firm the soil around the roots. Water them in and label the tray. Grow the seedlings on until they have filled their new containers.
When the weather is warmer, place the young plants outside in a closed cold frame. To give them a chance to harden off (acclimatize), gradually increase the ventilation over two weeks, until they are uncovered.
Plant out hardened-off plants in their final position in already prepared soil. Red cabbages should be 12in (30 cm) apart with 18 in (45 cm) between rows. If pigeons are a problem, protect plants with netting.