You can find these in stores specializing in casting and mold-making supplies. You can also sometimes find these in a well-stocked arts and crafts store. Most kits will contain two containers labeled "Part A" and "Part B." Sometimes, you have to purchase them separately.

Do not mix the silicone yet.
 Find a cheap, plastic food container made from thin plastic. Use a craft blade to cut out the bottom.Don't worry about how neat or jagged it is; this will eventually be the top of your mold.

Choose a container that is a little larger than the object you want to make a mold of.
 Remove the lid from the container. Cut several long strips of packaging tape, and lay them across the top. Overlap strip by about ¼ inch (0.64 centimeters). Leave a couple inches/centimeters of tape hanging over all sides of the container.Run your finger along the rim to create a tight seal.
Make sure that there are no gaps, or the silicone will pour out.
 Once you fill the container with silicone, there is a small chance that some may leak out from under the tape. This will kept prevent the silicone from leaking out all the way and ruining your work surface., Place the container onto a flat, stable surface with the cut/open side facing up. Place your items into the box, and press them against the tape. Do not let the items touch the side of the container or each other. Also, make sure that the design portion of your item is facing up, and the back is pressed against the tape.Flat-backed items work the best for this.
Wipe the items clean beforehand, if necessary.
 You will always have mix Part A and Part B together. Some types of silicone are supposed to be measured by volume, while others need to be measured by weight. Read the instructions that came with your silicone closely, then measure them accordingly.

Pour the silicone into the cup included with your kit. If your kit did not come with a cup, pour the silicone into a plastic, disposable cup.
You need enough silicone to cover your items by ¼ inch (0.64 centimeters).
 You can do this with a skewer, a popsicle stick, or even a plastic fork, spoon, or knife. Keep stirring until the color is even, and no streaks or swirls remain., Use your stirring utensil to help scrape out any excess silicone so that you don't waste any. The silicone should cover the top of your item by at least ¼ inch (0.64 centimeters). If you make it too thin, the silicone mold may rip., How long this takes depends on the type brand you are using. Some brands are ready to use in a few hours, while others need to be left overnight. Refer to the instructions that came with your silicone kit for more specific curing times. Do not touch or move the mold during this time., Once the silicone has cured and turned rigid, peel the tape away from the box. Gently slide the silicone mold out. You may notice thin "feathers" if silicone around your mold. If these bug you, trim them off with a pair of scissors or a craft blade., The items you placed into the box will be stuck inside the silicone. Gently bend the silicone back to pop the items out. It is a little like remolding ice cubes from an ice cube tray., You can now fill the cavities with resin, clay, or even chocolate (if the silicone was food grade). If you used clay, you can pop the pieces out while the clay is still wet. If you used resin, however, you must allow the resin to cure completely before removing it.
Purchase a mold-making silicone kit.
Cut the bottom out of a plastic food container.
Place overlapping strips of tape over the top of the box.
Fold the edges of the tape over the sides of the container.
Set the item(s) you wish to mold into the container.
Measure out your silicone according to the manufacturer's instructions.
Stir the two parts until the color is consistent.
Pour the silicone into the container.
Let the silicone cure.
De-mold the silicone.
De-mold the items.
Use the mold.
{"src": "wikihow-all_validation", "title": "How to Make a Silicone Mold3"}