1. Sunlight Map
Creating a sunlight map will help you to be able to decide what plants should be planted where. This will also give an indication of the soil conditions whether they will dry out (so you will need deeper rooting plants. Or stay damp so shollow rooting plants will survive.
2. Decide what you want - sensory- layout
Do you want a herb garden or a garden that you can enjoy the smells at night. Some plants will let off their perfume during dusk hours, others will during the day to find out more here is a great learning module by the bbc. http://www.bbc.co.uk/gardening/htbg/module2/
3. Annuals and Seasonal plants
6. Existing Plant life
7. Enjoying your garden