I'm just stepping to a larger world with mold making, and the possibilities are endless! Here's a basic how-to for molding and casting, in this case, an action figure.
The video above shows you everything you'll need to know, but here's the run down.
Step 1: Make a Box for Your Mold
Measure out the width and height of the object you're going to mold, then add at least 1" to the width. The height just needs to be a fair amount taller, no specific amount.
Measure and cut four sides and a bottom piece to make a box that fits your item. Hot glue the item to the bottom piece, then add the side panels with glue also. Once it's constructed, fill all gaps with hot glue to contain the silicone.
Step 2: Pour Your Mold
I used Oomoo 30 for the mold because it's affordable and easy to mix. It's a 1-to-1 ratio, so just pour equal parts into two cups, then pour them together into a large enough container. Mix REALLY WELL, making sure to scrape the sides and bottom of the container. Make sure that you have a consistent color throughout, with no swirling.
If you have access to a vacuum chamber, you can use it to remove air bubbles from the silicone. If you don't, try to pour it into the mold as a very thin stream to release some of the bubbles.
You can also use the vibration of a tool (sander, for example) to force the bubbles to rise.
Step 3: Demold Your Item
Once the mold is cured fully, remove the temporary foamcore box. Use a sharp blade to cut a serpentine path from the opening. Try not to tear the silicone, but rather cut it, so that it will fit back together tightly.
Carefully cut away any under cuts, and only cut what you need to, to remove your item.
Step 4: Prepare You Mold for Casting
To prep your mold for casting, spray the inside of it with a mold release agent.
Wrap you mold with blue tape, to hold it closed. Use a piece of wood on each side to distribute the pressure of the tape, which should be minimal. Don't squeeze the mold or it will deform... just hold it together.
Step 5: Pour Your Casting
I used a a resin, but there are lots of different materials to use for casting. The resin I used was a 1-to-1 mixture, just like the mold. Mix it well, degas if possible, then pour into your mold.
Pour a little bit, then gently and slowly roll the mold around to let air bubbles escape small areas where they might get trapped. Add a little more resin, then roll again. Repeat until you've filled the mold.
Step 6: Remove Your Casting!
After the cast is fully cured (which will probably take longer than you want) remove it from your mold just like you did with the original piece!