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First, you need a mold made of plaster. The plaster mold absorbs the water from the slip.
Fill the mold with slip. As the mold absorbs the water, the level of the slip in the mold will go down requiring you to top off the mold.
OK. Here's the trick to it all: knowing when to dump the excess slip.
If you leave a mold filled with slip too long, the ceramic piece will be much too thick and heavy.
If you don't leave it in long enough, it will be too delicate and squash and tear as it comes out of the mold.
For my small figures and pieces, I like to leave the slip in the mold for about five minutes before I dump out the excess slip.
On the bigger items, I watch the mold for an eighth of a inch skin to form around the lip of the pour holes before I dump the slip.
Now, a few pieces call for the piece to be poured solid. These include things like plates, bowls, and other utility items. These molds you just keep filling until they stop sucking up slip.