Designing annual flower beds can be simple, as long as you know what you want: Do you want everything to bloom at once and then die out so you can start over, or do you want to mix in perennials, shrubs and even trees so you only have to replace a strip or two of annuals once they die out? In any event, there are some basics of garden design that apply to both strategies.
Prepare the flower bed by using a pickaxe to loosen the soil to a depth of about six inches. Remove any weeds or other vegetation and then mix in topsoil and peatmoss. Rake smooth.
Lay in the border by digging a trench between your newly cleared flower bed and the lawn or walkway. Place the border materials into the trench and push down, and then push each piece up against the next for a tight fit. When done add soil to the inside of the border area and pat down to secure border materials in place.
Randomly place a few decorative boulders into your flower bed. They will help give it a sense of dimension. You also might want to consider a garden sculpture or even an antique wheelbarrow or wagon wheel.
Plant your flowers. Be sure to pick flowers that will grow in your climate zone. Plant the taller flowers in the back, and the smaller ones in the front. You should have at least three to five rows of plants, tallest in the back, shortest in the front, regardless of how wide your flower bed is, to give it texture. Try to use flowers that complement each other, such as foxglove in the back and snapdragons in the middle and alyssum in the very front. If you want to mix in perennials plant larger specimens in the back, leaving the front strip for annuals that can easily be replaced when they die out. For example, in Southern California, a row of petunias, planted in the spring, will die out by summer; at that time, plant marigolds, which will last through the fall. If you plant in rows, make sure they are wavy rather than straight. And always plant in clusters, so that you have at least three to give plants of the same kind in one spot.
Sprinkle snail pellets or powder in circles around each plant. Many a gardener has planted a beautiful flower garden, only to see it destroyed within a day or two by an invasion of hungry snails.
Make sure you keep your annual garden well-watered. The soil should be moist at all times. Never let the garden dry out; some annuals are easy to kill and must be closely monitored, especially on hot, dry days. But also take care not to overwater. Watering too much can cause root rot and eventually kill the plant.
Buy any type of flower fertlizer, such as Miracle Gro, and use as directed. You'd be surprised at the difference it makes in keeping your flowers in bloom.