Now you pour the silicone that will become your tentacle. It's important that this material's thin enough to percolate into every bit of the mold and let air bubbles ventilate out. It also helps if it has a reasonably long gel time to allow every bit of air to escape. I went with a 50-50 mix of Smooth-On's Dragon Skin and their Eco Flex (which means mixing up a cup that's 50-50 of each material's part A and another of their part B's before combining everything for the final pour). If you want to make things go faster, you could always use a vacuum chamber.

Pour in a thin stream, elevating the cup about a foot off the mold to help pop any bubbles trapped in the silicone from the mixing process. I've found that half filling the mold with silicone before inserting the core can help keep bubbles from collecting at the top and giving the tentacle structural flaws, but you've got to be careful to insert the core slowly to prevent it from trapping big bubbles in its nooks and crannies.

Wait overnight for everything to cure. I recommend pouring up a little swatch of silicone out of the same batch into a little dixie cup so you can check whether everything's curing without disturbing the mold itself.