The soil I dig up from my back yard and for fiber I use either shredded wood fibers that I get free from a local tree service or I buy a bail of straw and use my lawn mower to chop the straw. These are typical wood fibers.
Step 1: Make Test Brick
The next step is to mix a handful of straw and some clay and water to make a test brick which you will test for strength. Make a 4"x4"x4" brick and dry it completely and see if it will support your weight then try to scratch it with your fingernail or break it up with your hands. If it supports your weight and does not crumble when scrapped or twisted by hand you have a suitable building material.
There are several ways to mix mud and fiber. I prefer to do it with a rototiller by simply tilling the soil adding water and fiber and tilling to form a thick mud. You can also do this with your feet, or you can put the mud on a small tarp and mix the soil, water and fiber with your feet and roll the mud by pulling the edge of the tarp to put the dry soil on top of wet mud.