If you wish to mold only oone side of an object which is flat on the other side (for example a low or high relief sculpture), you can realize a block mold, much simpler to realize that the mold in two parts.
In a cutable bowl, stick the object at the bottom, its flat side downward. If the object is too big, you can also build walls around it with Plastiline on a board. It is important to stick the object to avoid it to move when you will pour the silicone. Fill the bowl with fluid silicone taking care that the object is fully covered. Let it take and the cut the bowl or remove the walls.
It only remains to pour plaster into the mold to realize the epreuves and to correct possible small imperfections.
If the object to be molded is of cylindrical or conical shape, you can realize a mold sock. Stick the sculpture (with double face adhesive tape for example) or use the fluid coat to fix it, but attention not to move the sculpture.
Pour the impression coat taking care with a brush that it penetrates into the hollows of the object. Let it take and then apply the thixotropic thickness coat. Create a more important thickness of silicone where the cut which will allow to remove the mold will be (behind the sculpture). It is called the crest. Smooth the silicone with the smooth edge of a sponge.
Once the siliconeis taken, cut an opening in the thick part behind the sculpture, rather in zigzag to avoid that the silicone risks to tear.
You can then realize a screed in two pieces with plaster bands to solidify the mold, but attention, this one might get used quickly (it will probably last 2 or 3 editions only). If you wish a more long-lasting screed it will be necessary to realize it in plaster with oak.