Before you mold with polymer clay you need to condition it. That simply means that you will knead it in your hands until it is smooth. You want to continuously fold it, squish and roll it to break it in and get it soft. To check if you are done conditioning you should roll the polymer into a snake about the size of your thumb. If the polymer cracks when you bend the snake in half you should continue working with it to get it softer and more pliable.
The approximate amount of time to knead is to think of how long it would take to blend two colors together. Then roll it into a ball and dust it generously with cornstarch before pushing it into the mold. If you have excess clay after filling the mold you can either cut it off with a pair of scissors (careful not to clip the mold), pinch it off, or run a straight edge spatula across the top to remove the excess, then smooth it again.
After you have pressed your polymer clay into the mold, you can simply pop the piece out by gently putting pressure on the sides of the mold and flexing it out and up. Or you can place the filled mold into the refrigerator or freezer and let it harden for easier de-molding if you are using a mold that is deep, highly detailed or has undercuts. Let the finished piece come to room temperature on waxed paper before baking. You must bake polymer clay (follow package directions) or it will remain soft, it doesn't air dry.
If your clay is too firm even after conditioning you can add a tiny amount of petroleum jelly (Vaseline) to it. Be careful, it is easy to overdo it. If the clay becomes too soft you can let it rest overnight or place it in the refrigerator for an hour or so to firm it up.
You can flatten the clay between two pieces of paper if it becomes to greasy or oily by adding to much Vaseline to soften it. This will soak up much of the excess. Plain newsprint works best.
You can paint a baked piece with acrylic paints.
Never dust the inside of the mold.