I have been using silicone molds and resin casting for about two years now, and I have found it to be a great way to make durable and reproducible pieces for art. So here's a few tips and tricks which will help you get started on casting while hopefully limiting errors and wasting resources.
Making the Master - First thing you are going to need for casting is a master sculpt. You can use a pre-existing model, such as a toy or sculpture, or you can make your own using clay. If you're going to make a clay sculpt, make sure to use a sulfur-free clay. If the sculpt contains sulfur, it will react with the silicone and prevent it from curing, and you'll have a sticky mess to deal with. For clays I highly recommend regular sculpey, plasteline clay, or a wax-based clay that can be reused. DO NOT USE POTTER'S CLAY. That contains sulfur.
Other things that play well with silicone:
-resin and most plastics
-most natural materials (bone, rock) provided you can tell they are sulfur-free.
Things that don't play well from my experience:
-Wood- maybe it's my bad luck, but most wood I've tried to cast has resulted in a mess.
-Silicone - silicone sticks to itself, so you need lots of release agent. If you can, avoid silicone on silicone.
Here's an example of a master sculpt I made for my wasp mask.
The collar around the sculpt is to give a lip to the mold, allowing it to catch extra resin during casting and help stabilize the mold. The squares in the collar are keys. Keys ensure a snug fit for your mold so the castings are consistent.
Making the Silicone Mold - When it comes to silicone molding you have two main choices: tin cure or platinum cure silicone. I highly, highly recommend platinum cure as heavy exposure to tin can poison you, even though platinum cure silicone is more expensive. My favorite source for silicone is Smooth-On, as they offer silicone in various hardness and curing speeds. Silicone comes in a 1:1 by volume ratio, with A and B components. You want to pour A and B into separate cups, then combine the two and mix evenly. Pour the mixed silicone over the master sculpt, starting at the top. carefully brush silicone into all the nooks and crannies, and keep bringing silicone up from the bottom back to the top to limit waste. Keep applying silicone until it begins to 'kick' - the silicone will stop being stringy and start breaking. Make sure that this first silicone coat has as few bubbles as possible and captures details.
The second coat of silicone is used to strengthen the mold. You can thicken the mold using a thixotropic agent, or just add corn starch. If you're using thixotropic agent (also available from smooth-on) use only a TINY amount; one bottle will last you for decades. For corn starch, just add some until it gets a bit thick. You want to have about 3 layers total, or until the original sculpt is well obscured.
Mandible sculpt with silicone mold- I didn't get any of the mask.
Making the Mold Jacket - Now you have a silicone mold which has cured; time to make a mold jacket. For the mask mold I used Plasti-paste, which is really easy to apply. However, it is pretty dang pricey compared to plaster. I don't recommend using plaster strips for molds, as it's too pricey. You'll need a scale for Plasti-paste as it's a 66:100 ratio by weight. Mix the two together and quickly apply it to the mold. It will cure and get very hot very quickly.
Mold jacket with cured (right) and uncured (left) halves. Make sure to divide the halves of the mold with a line of tinfoil and grease it up with petroleum jelly.
Finally, Casting! - After all that hard work is done, you can finally make resin casts. Before casting, spray a light coat of release agent into the mold, or brush in some and let it dry. I make my own release agent by dissolving petroleum jelly in naphtha. (Be careful with naphtha, as it's quite volatile to organics. My workshop buddies say otherwise, but I think it's worth 30-40 to get the aerosol can from Smooth-On). Next, mix the two components of the resin (once again, Smooth-On is recommended, especially ONYX) and pour into the mold. rotate the mold with your hands until the resin evenly coats the mold and begins to cure. Repeat around 3-4 times depending on how well you rolled the mold. This is called "slush-casting". Alternatively, if you're making solid parts like claws or accessories, just pour the resin to the top of the mold. Wait until it cures, then carefully take it out of the mold. And now you have a finished piece!
And that's about it! If you have any questions, or you feel like I left anything out, please feel free to add your own tips and tricks for successful casting.