We'll cover the two different ways that this can be accomplished. I've let hides sit in the water for several days always making sure to change the water every day. You'll notice that any excess blood will go into the water as well as some hide glue. A scum will start to appear on the surface over time and the water will cloud with whatever else comes out of the hide. Do not be tempted to add soaps or softeners as some have suggested to help clean the hide.
Plain water does just as well without the chemicals.
We're talking pure brain tanning here and not partial or plain ol chemical tanning. If the hide is a large hide, I will take the hide over to my stretching rack and hang it up. I have 4 such frames connected together to form a box with an added piece for my entrance into the middle of the box. This allows me to stretch 4 hides at one time. My frame is made from 2X6's for added strength.