Use starter plants from a nursery, or start your own plants in starter pots. Let the plants grow alone until they are beginning to bud before you transplant them.
Select plants with similar growing requirements. All the plants in one pot should have similar soil needs, pH balance preference, water, feeding and light requirements so that they will grow together harmoniously in the same environment.
Choose plants with a color scheme that you like. You might like something monochromatic, such as a striking arrangement with all red blossoms. For something harmonious, go with soft mixtures of similar colors, such as pink, blue and purple. For something more vibrant, use primary or secondary colors that are opposite on the color wheel, such as yellow, red and blue.
Select some plants to stand tall, and some at medium height, and some to hang over and trail the edge of the pot gracefully. This variation will make a fuller display and prevent front plants from obscuring those in back.
Choose a decorative container for your plants. Be sure it is wide enough to house the amount of plants you wish to put in there comfortably, and deep enough to give the roots room to grow and space for air and water. A good way to judge if your planter is large enough is to place the starter containers directly into it to see if they fit. Add some crumpled newspaper to the bottom so the soil line of the starter pots will sit at where the soil line will be in the container.
The plants should all sit comfortably, with one to four inches of space between each one. If you can't squeeze them into the container while they are still in their starter pots, you won't be able to squeeze them in after removing them from the starter pot without the risk of overcrowding and damaging the roots.
Fill your container with a layer of gravel or Styrofoam packing peanuts to promote good drainage. Add your growing medium, such as potting soil, compost, or any mixture recommended for the plants you have selected. Fill the container halfway up, so that when you place a starter pot in it, the soil-line of the starter pot meets where the soil-line of the container will be.
Place the tallest plants in the back or the center. Plants at medium height or that grow wider rather than taller will go in the next row, or, if you are using a round container, will surround the tallest plants. Plants that hang over and trail should be placed around the edges of the container. Experiment with the arrangement until you are satisfied.
Gently remove the plants from their starter pots and place them into the container. Add soil around them to help hold them in place and cover the root ball. Water the container. After the soil settles it may expose the root ball. Add more soil to cover it.