Prepping the mold is the first order of business. Depending on how long it’s been since the mold was made, it might need to be cleaned again to ensure that no contaminants such as dust and stray hairs have found their way onto the negative surfaces of the mold interior.
Once the mold pieces are clean and dry (if they’re stone molds) they will need to be sealed and released; if the mold is made of fiberglass or other resin, it probably doesn’t need to be released, though it’s never wrong to release a mold if you have any doubt as to whether the silicone will stick or not without it. If you will be casting silicone into silicone, this is very important: Tin silicone can be cast into a mold of either tin or platinum silicone (the mold must be properly released, of course) but platinum silicone can be cast only into a platinum silicone mold, not a tin silicone mold.