When using dry mixing materials for making pottery clay you will first need to mix the dry materials to ensure even distribution of all different types of materials.  If you skip this important step the clay will be in layers and will be extremely difficult to mix properly once the water is added.  There are a couple of good ways to mix the dry materials.  There are commercial clay mixers that can mix clay wet or dry, however they are very expensive and if you mix dry materials, it makes quite a mess with dust. Another way is to put the materials in a 5 gallon bucket with a really tight lid and shake it up.  You can even roll or kick it across the floor to accomplish this. If you are making a large batch of clay you can do this is any type of barrel that has a lid. When you add water, you will actually be adding more than necessary, but you will want to add it slowly to prevent dust from flying up in your face. You will then need to let it sit for a while to let it soak into the dry clay.  It is a good idea to let it sit overnight so that it has time to thoroughly saturate the clay, then you will mix it.