When I was in college studying pottery we mixed up our slip using wet clay every time we got to class. Later I learned mixing slip from dry clay is much much easier and the slip is much smoother. Here's how I mixed up some red and some white slip.
Just for fun hold the bowl up to your ear after adding the water, you can hear the water being absorbed into the clay. I know, I'm easily entertained, but there is something kind of magic about hearing the air in the clay being displaced by the water being absorbed. I set the bowl aside for about two hours or longer without stirring or mixing it.