Before any design work can be done your garden needs to be surveyed, to be able to draw accurate scaled plans.
A variety of things must be established, including the boundaries, house, windows and doorway measurements, any level changes, the aspect, the soil type, the views, existing services, drains and man hole covers and to establish any issues or problems.
In smaller fairly level gardens I can often do the surveying myself, but in larger gardens or if there are complex level changes, a professional survey may need to be commisioned.