Mix together parts A to B mixture into a plastic bucket. Read manufacture's instructions. Many mixes are a one-to-one ratio of the two parts and some use slightly more of one part. Using a rubber spatula or wooden spoon quickly incorporate both parts.
Pour mixture into mold. The working time for the foam rubber at room temperature is depends on the mix you purchase. Read the manufacture's instructions for working and curing time.
Wait until the foam rubber is set and cured. The foam will expand many times in volume. Pull the finished foam rubber out of the mold. Or if you are using it as a filler, leave it alone.