Choose an item that does not have undercutting, and/or realize the undercutting could affect your final outcome.

Condition the polymer clay you are using to make the mold. On a sturdy surface, press the item straight down into it to get a good impression.

Pull the item out and away from the clay as gently as you can and bake according to manufacturer's directions.

To make a positive;

Allow the mold to cool. Lightly dust the interior with talc, flour, or corn starch. Be sure to get into any and all crevices.

Press conditioned polymer clay into the mold. Gently pull the clay oout by releasing the edges of the clay impresison first and allowing the rest to follow.