The first step in your project is to check your local council requirements to see if you need to comply with any regulations. If you're planning on working with a large amount of earth it is also recommended that you consult an engineer.
Next is to draw up on paper your desired retaining wall. If possible plan using readily available sleeper lengths to avoid having to cut the sleepers. Check with your local Mitre 10 which lengths they have in stock.
Make a list of the number and size of sleepers that you'll require. Allow for the first sleeper to be embedded around 100mm into the ground.
Make a list of the number and type of HardAz posts that you'll require. The space between posts is suggested to be around 1200mm. Around half the length of the posts will be below ground level. For example, a 900mm high retaining wall will require 1800mm posts.