When the mold is fully cured, I use a universal mold release again for inside the concrete rubber mold. Letting that set for about 5 minutes. Using a concrete patch repair mix is what I use next. Mixing that until it is a puddling like thickness with it. Casting that into the mold then a bit of shacking the mold to level the concrete out for a smooth bottom look, and that will help get any air bubbles out. I would let the mold set for about an hour. Then if the concrete was solid enough, I will tip it up and have a fan on it for the next 2 - 3 hours. After that amount of time it would be ready to pull from the mold.