Using a soft mechanic pencil I draw my design directly on the plaster.I then carve my design into the plaster, keeping in mind that anything carved into the plaster will end up as a raised area on the final slab.
like to carve the mold while the plaster is still a bit damp since it is softer at this point and easier to carve. It also leads to less dust. Still, there are drawbacks to this method. When transferring your design, it is very easy to mar the surface of the plaster with your pencil or scribing tool. The ink from markers will bleed a bit, and pencils sometimes do not draw very well on wet plaster. Also, if you use a paper template, the paper will become damp and warp a bit as you trace your design. If you carve your mold when the plaster is completely dry, take precautions to minimize your exposure to the dust and use a mask.
Once my design is carved I make sure all the plaster dust has been cleaned out of the cavity since any plaster contamination will ruin a final ceramics piece! There will still be a bit of plaster residue left, so I press my first tile with some scrap clay.