The first steps obviously involve a certain amount of design and this will depend on your site, required garden size and other variables that you consider important for your garden. The major variable that I can guide you on will be height of the garden. I have made this garden about waist height, so about 1m high. This is primarily so one can bend over and reach plants toward the rear of the garden. You can make yours any height, however the post hole depth that you will need to dig should be at least the height of the garden. Essentially we are trying to create a retaining wall so be very careful with your design - you don't want to harm anyone from a collapsing raised garden!
Also before you start, ensure you have enough branches. I ended up using more than I could have imagined so give your trees a good trimming! Also if you are building next to a fence (as I have done) ensure that you place a polythene sheet between the fence and the soil to avoid rotting of the fence.