Plan the Structure First
First, mark out your bed or border with nontoxic spray paint, sand from a bottle or a hose. Then make a rough paper plan by scaling down the length and width of the bed to, say, 1 inch on paper to 10 inches on the ground, or 1 inch to 20 inches. Then start plotting areas for the main structural plants, such as shrubs and trees. Check their heights and spreads to ensure they will have space to grow where you want them. Then draw circles with diameters that represent the spread of each plant on your paper plan. Alternatively, draw circles with sand from a bottle or nontoxic spray paint, marking the positions of the plants on the actual bed.
The next step is to plan the planting around these large specimens. Make sure perennials are a good distance from the central stems of trees and shrubs; spring bulbs that flower before the canopies open can be planted closer.
When planning a tree in a border, take note of its spread, which will affect the light and water available to plants beneath it. The light leaf cover of this maple allows dense planting below.