For a steamer juicer to work you only have to dice the fruit in large pieces, you don't have to take out the cores or bruised parts. The steamer has 4 parts, in the bottom one you put boiling water (make sure it doesn't dry out), in the next you will capture the apple juice, in the third are the apple parts in a sieve-like basket, and the top layer is the lid.
We let the apples steam for almost two hours and ended up with nearly clear apple juice. Almost 4 liters for 5 kg apples is not a bad yield as I may say!
For other recipes and a better how-to please check out the "steam juicer and food steamer recipe book" by Mehu Liisa Products: link (a PDF)