After removing all the weeds and large stones, add a balanced fertilizer and dig in lots of organic matter, manure and compost. Leave it for a couple of weeks to settle down and then level the soil.
This will give you time to choose your plants. Some favourites like campanula, aquilegia, lupins and violas come immediately to mind but the final choice will depend on such considerations as the type of soil and how much sun the bed will receive.
As for the colour scheme, decide whether you want a loud selection of bright colours, relaxing pastel shades or perhaps a pick and mix collection of contrasting colours.
Imagine it as a painting, is it an orange and yellow Van Gogh you're after, an impressionistic Monet or maybe a modernist Hockey. You don't want to have any straight lines but it's probably wise not to be too chaotic, on the other hand who wants to be bland?
Go to a few shows or pinch ideas from your neighbours' gardens, see what looks good and grows well. Jot a plan down on paper or on a computer, moving the elements around to the greatest effect. Avoid planting singly, use blocks of plants for continuity of colour and the greatest impact.
Note flowering times. You don't want half the border in bloom and the other not. So mix plants to have a continuous show all season throughout the border.
Foliage is also important, if you have a plant with rich foliage put it next to one with sparse - some plants with exceptional foliage can be included on that basis alone.
Include a mixture of evergreen and deciduous shrubs. Enliven the senses with textural foliage such as downy lamb's ear and fragrant flowers, decorative stems or bark. Use plants that attract insects and butterflies such as Sedum spectabile.
When you're ready, place out the plants, still in their pots, to see whether they have enough room and you are completely happy with their positioning.
Plant the shrubs first, they need time to establish and herbaceous plants can be moved more easily if you decide they are incorrectly located. Taller plants predictably should be at the back of the border while smaller plants and ground cover are at the front. If there is an fence or rear wall, climbers will provide an attractive backdrop.
In the back row they'll be delphiniums, verbascum, heliopsis and aruncus (goat's beard). In the middle, phlox, peonies, dahlias, rudbeckia and pyrethrum, while at the front can be gaillardia, scabiosa, chrysanthemums and asters.
Bulbs, including corms, rhizomes and tubers, are perfect to fill any gaps. When planting remember that they hate bad drainage, so always plant on a bed of sharp sand or grit. Not all are complete sun worshipers, some summer flowering bulbs like lilies prefer their roots in the shade, so plant them so they grow through low shrubs.
After firming the plants in, give them lots of water and mulch. Continue to water and deadhead over the summer, only the larger ones will need staking and it will be about five years before any need dividing.