Find a bed location that is in full sun with well-drained soil. Work organic compost into the soil with a pitchfork.
Dig holes in the bed 12 to 24 inches apart and as deep as the plant's pot. Plan out the location of each plant to create the look you want to achieve. Plant the same-colored petunias together to create lines, waves or checkered patterns in the garden. Or plant a mix of colors so they all blend together. Set each plant next to the hole it will be planted in.
Loosen the pot from around the root system by gently pressing on it until the plant slides out. Set the root system of each plant into its hole. Cover the roots with soil and pat the soil down with your hands. Water the plants thoroughly immediately after planting into the bed.
Water the plants every morning before the sun is at its hottest. Apply a liquid fertilizer every 10 to 14 days by following the manufacturer's instructions. Fertilization will help to maintain the plant's health as well as promote blooming and spreading.