Smooth on videos
There is also a section in Shepherd Paine's Diorama book on molding parts.
Will it be open faced or a 2 part mold?
Open faced you can tack the part to a flat base with sides that are high enough that the silicon will be able to cover the piece, for small pieces the base of a paper cup may work.
Some folks use Lego's for building the box and lining the bottom with clay, you can also use foam board or anything that will contain the silicon, you don't want to make a mold of the table or floor.
A 2 part mold takes more engineering with the piece embedded in the clay with vent lines and registration marks added than once the first part has been molded it gets flipped, the clay removed and a mold release added so the silicon doesn't stick to it's self than pour the second side.
Once done open the mold and remove the part and you should be ready to cast.
Depending on the type of silicon and if the proper amount of catalyst is applied the cure time may be in the area of 24 hours, some cure quicker.
Depending on the type of silicon you are using "tin of platinum" based you will have to be careful of the type of clay you use.
See the spec sheets on the Smooth on site.
Eric...