Start filling the mold with little pieces of clay, making sure that you’ve pushed it down into the smallest and most detailed areas of the mold. If the clay sticks to your finger and won’t stay in the mold, just roll your finger off the clay, rather than pulling straight up. Add more clay until the entire area is filled with clay.
Once you filled the cavity, press down with your thumb or fingers to make sure all the bits of clay are fully fused together and that you removed all the seam lines of the clay pieces.
Use a blade to carefully slice off the excess clay. It’s better to take several shallow passes with your blade rather than one big slice. If your molded piece pulls away from the mold, just push it back in. Sometimes the drag of the blade can pull the clay out of shallower areas of a mold.
Use a needle tool to dig out excess clay from hard to reach area areas. Use your fingers to gently “sweep” the edges of the clay toward the center, away from the mold. This ensures that there isn’t stray clay around the edges of your casting.
If your molded piece is fairly shallow and doesn’t have a lot of fine projections, you may be able to easily remove the polymer clay from the mold by turning it upside down and flexing the mold. The clay might very well just fall out easily with no distortion.
If it needs a little more coaxing, avoid using a craft knife or a needle tool to do that, as it’s usually going to create a problem by distorting your polymer clay. Instead, smooth a piece of clear plastic sheet onto the clay, adhering it. Then invert and try flexing the mold again. Use the extra “suction” power of the clay being stuck to the plastic to help pull the clay out of the mold.
If that doesn’t work, get a clean piece of glass or glossy ceramic tile. Invert the mold (and clay) onto the tile. Press down firmly over all areas of the mold, making sure to adhere the clay to the tile. Now try lifting the edge of the mold, flexing it as you pull it off, carefully checking to see if the clay is staying stuck to the tile.
But if you can’t get the clay out of the mold without distorting it, all is not lost; go ahead and cure your polymer clay right inside of the mold.