Spatial vs. Planting Design
As you approach garden design, it's important to understand two aspects: spatial design and planting design. Spatial design can easily be overlooked, by jumping ahead into planting design. It's like writing a sentence without understanding the overall structure of the paper.
This is the overall spatial arrangement (or structure) of your garden. Spaces (or rooms) are defined then arranged so they relate to each other in the appropriate way for a client. To figure out spatial design one would go through a series of functional diagrams (bubble diagrams) and concept drawings.
Once the spatial design is finalized, plants and other hardscape elements are used to define those spaces. Planting beds are formed to reinforce the spaces you created. It is within these beds that the magic of planting design occurs. Planting areas should not be placed willy-nilly, but always with purpose.