Before randomly selecting flowers that look pretty, it is important for you to know how the plants are going to bloom and when. Some plants bloom in the early spring and last through a good portion of the summer. However, some plants don't bloom until mid-May and then only last for a week or two. Be sure to plant flowers that bloom when you most want your garden to come to life. If you're looking for a design that looks in-bloom for most of the year, be sure to alternate flowers and their seasons so that the garden doesn't look like half is alive and half is dead depending on the season.

Some may find greenery boring, but it is important to have a solid base for your garden so that it doesn't look sparse or ill-planned. Choose greenery that complements the types of flowers and plants that you have chosen, mixing in tall ornamental grasses and low, ground covering crawling plants. These will help frame the beautiful plants you chose and give a sense of completeness to your garden. 

Arrange the flowers and plants by height as well. You wouldn't want to place a tall rosebush at the front of your garden if you are also planting short, delicate flowers, or no one will see them! Read the instructions on each plant or their seed packet to make sure you understand how high they are expected to grow, and then plant them accordingly