Select a space between the tulips. Gently dig a hole 1" deeper than the petunia plant. Sprinkle a little fertilizer and water the hole. Loosen the roots of the petunia and press the plant into the hole. Backfill and gently press the soil. The root ball should be completely covered in soil and be slightly lower than the surrounding soil. Cut back the spent tulips, but don't cut the leaves as they provide nutrients to the tulips for next year. The petunias will soon cover the tulip leaves.