I always make such sketches to see what will the border look like.
You probably know well what climate conditions are in your garden and what kind of plants grow the best in the type of soil you got. Make sure you have perennials, biennials and annuals in your border. See when they are blooming in your area to have colour throughout he season. Sometimes the perfectly planned flower border fails because of the different flowering time than it is supposed to.
Add some plants with interesting purple, silver or variegated leaves. Have plants with different foliage and flower texture.
Repeat some plants along the border to give it the rhythm and harmony. Too much of everything may look chaotic.
Measure your border and transfer the measures on the paper. If you find it easier than drawing make differently coloured circles to represent the plants. Make sure they are on scale with your border plan. It is easy to move the circles around the border until you are satisfied with the look.
Start with bigger perennials, shrubs and roses. Usually they go in the back of the border. Then, mix in medium and small sized plants. Low growing or creeping plants go on the front. Plant the plants in drifts that interfere to achieve more natural look. Plant at least three plants in one drift; uneven number of plants looks more natural, so you can plant three, five or seven plants in one drift. Transfer your plan on the ground. Trace the plant position by using the sand or kitchen flour. Marking the drifts this way will help you visualize the flower bed and estimate how much space you need for single plants.
Large plants like e.g. Crambe cordifolia or Perovskia can be repeated or plant only one for impact. Mix in the grasses; they bring movement and interesting shape.You will also need fill-in plants like Verbena bonariensis or Alliums to fill in the gaps between other plants.
And here, there is a watercolour of one border for shade.
1. Anemone 'Honorine Jobert'
2. Veronica longifolia 'Pink Damask'
3. Hosta 'Elegans'
4. Chasmanthium latifolium
5. Astilbe japonica 'Köln'
6. Platycodon grandiflorus 'Fuji White'
7. Tradescantia 'Karminglut'
8. Geum coccineum
9. Gentiana septemfida var. lagodechiana
10. Heuchera sanguinea 'Leuchtkäfer'
11. Carex morrowii 'Variegata'
See more planting schemes here: DECORA