Silicone is a polymer that comes in many different forms from silicone oils to rubber like compositions. Silicone rubber comes in many different shore hardness levels. There are two basic types of silicone rubbers, tin-cure (condensation cure) and platinum-cure (addition cure). The difference between the to is the catalyst that cures the rubber. The main practical difference is that tin-cure rubbers tend to last from 3-5 years in storage and platinum-cure rubbers can last for many decades in storage.
It is very important to follow the mixing instructions that come with the product exactly. For this reason I prefer weight based mixing standards over volume because it is much simpler to weigh out the required amounts into the same container than to mix from two containers and try to get all of the material mixed together.
To prepare the silicone for pouring, it is important to degas the silicone in a vacuum chamber before pouring. This ensures that there are no air bubbles trapped in silicone before pouring. Ideally the mold would also be degassed after pouring to make sure that silicone has reached all points in the mold.
To degas, place the mixing vessel in the vacuum chamber, turn on the vacuum pump, and close the valve on the top of the chamber. Watch the level of the bubbles and use the valve to ensure that the vessel does not bubble over.
For best pour results, pour the silicone into a single place in a thin stream at the lowest point of the mold. This help prevent bubbles from becoming trapped during the pour.