To demonstrate this, I used a little rabbit that's been hanging around on my mantle for a while. It was a good example of an item that's going to make a good mold, since it doesn't have any kind of parts that double-back over themselves.
To make the silicone, mix according to the directions...
Turn the piece over and make sure that the mold is open at the bottom. In this case I had to kind of push the silicone away from the neck and the tail to make sure the rabbit would be able to come out of the mold when it cures.
(You could just make little spikes by hand, but this makes them all uniform...And if you use chocolate to make the shapes, they'll be really smooth and sharp.)
I did the same thing with the little pyramid, then pressed the silicone to make sure that the base of the pyramid shape would be square.
The materials that work best in deeper molds are a little stiffer, so gumpaste would work better than fondant, etc. It just makes it easier to pull the molded item out of the cavity.
I used some melted chocolate to fill these molds, then put them in the freezer for a while until they hardened up. I also tried it with gumpaste, and that worked fine. You'll probably have to trim the bottom edge of the shape after taking it out of the mold unless you're able to level off the material exactly with the cavity opening when you're filling it.
The melted chocolate gives you the best edges and the sharpest shapes, but you also have to make sure that the chocolate gets down into all of the mold. If it doesn't it can leave an air bubble that will ruin the shape. Fill it bit by bit, using a toothpick to spread the chocolate out in the mold cavity, then give the mold a good whack on the counter to try to dislodge any bubbles.
Next- Two-piece molds