Slush casting is easy. You take the slush (the material the item is going to be made out of) and then you pour it in a pourous mold (such as plaster of paris). You let it set for a while and the mold will leach out the water, etc. from the slush. You then pour out the extra slush and you are done. Though this won't work well with resin, generally this is how latex masks are made.
To use resin, and I think this is what Evan was talking about, you want to roto-cast. Pour a small amount of the resin into the mold and then slowly rotate it (i.e. "slush") around until the resin has hardened.