It's better to leave the shredded paper in water overnight so that it breaks down more easily, but you can do it without that. Put some shredded paper in a blender, add water (more water than paper) and blend it all together. You can do this manually using a pestle and mortar.
When you don't see any clumps of paper or unshredded paper, you can stop blending it. You now have paper pulp.
Dump this paper pulp into the large tub and add water so that it covers the mold and deckle (one on top of the other) horizontally.