The secret to creating a professional looking seasonal container is to leave no open space between young annual transplants. When planting annual flowers in the ground generous spacing allows plants to grow bigger. However, planting in pots is different because the limited soil means plants need supplemental water and regular feeding and therefore more plants can be supported in a smaller area. A couple plants may be smothered by larger plants after a few weeks, but by then the lush growth will be a sight to behold.
Container Gardening Complete by Jessica Walliser (Cool Springs 2017) covers the basics of growing plants in pots and also has many fun “recipes” for thematic containers such as “butterfly garden tubs” and building a “beet box” from repurposed CD storage boxes.