Along with your other brassica, Brussels Sprouts are heavy feeders. We amend our gardens heavily with locally composted manure each season. Consider placing your plants out as soon as the weather, ground, is fit for cool weather crops. I space mine about 18″ apart. For best production, plant in full sun. I also mulch around my Brussels heavily with grass clippings after planting which seems to increase production.
I highly recommend covering your Brussels with row covers to mostly eliminate pest pressure. Nothing is more discouraging than having your cabbage, Brussels, and broccoli gobbled up by cabbage worms. Covering with a row cover keeps the white cabbage moth from laying their eggs on your plants. Problem solved. If stored properly, row covers will last for years.