Allow at least 6-12 inches in diameter per plant in the ground or in a pot. Keep lettuces well weeded to prevent the weeds from interfering with the lettuces growth. Part sunshine keeps them cooler longer and prevents early bolting or flowering. Plant them early or late in the season!
Always water low to the ground about once a week and never use sprinklers to water your vegetable garden as wet leaves can promote rotting leaves. You can cut the leaves when they are about 4-5 inches tall and the plants will grow new leaves within about 2 weeks. You do not have to harvest them all at once, just leave what you are not ready to use and cut them as needed.