Silicone rubber needs to lose solvent vapors to harden up, so working in closed molds is difficult. These examples were done with open plaster molds....
Step 3: Technique: Drawing on a Flat Non-stick Surface
You can do flat designs, or flat parts for larger designs, by extruding silicone on a flat non-stick surface. Polyethylene plastic is a good material for the flat surface. You can stretch trash bags over a board as a cheap surface to work on. I prefer a rigid polyethylene kitchen cutting board, like this one I got at Sam's Club.
After the silicone hardens, you just peel it right off.
The parts below are mostly fins for a fish sculpture I made.