Prepare planting bed by removing weeds and incorporating granular fertilizer such as blood meal into the top 6-8” of soil. Then plant seedlings 12-18” apart in rows that are 24-36” apart to allow adequate room for this very large plant. Once planted, keep your soil evenly moist (do not let it dry out!) during the heat of late summer/early fall.
Help your cabbage seedlings get as large as they can prior to late October by fertilizing weekly with a dilute mix of liquid fish emulsion and liquid kelp (aka seaweed), and then provide winter protection such a low tunnel hoop house or tall cold frame.