Chop into small pieces. Heat carefully so it doesn't catch fire-maybe a heat gun on low? Heat up your mold-the plastic will freeze when it touches the mold if it gets a chance. Pour the thick glop of plastic into the mold. What kind of mold are you thinking of using? Without an injector of some kind, it will probably need to be an open-faced mold, or you would have to heat it and let the mold keep melting the glop of plastic (and have a way for the air to escape). What are you thinking of making the mold out of? If you want good detail, the mold is going to have to be heated, leaving out any plastics.