Attach the flat side of your part to the bottom of your container with thread or wire so it doesn't move or float up. Leave approximately 1 inch of space on all sides so the walls of the alginate mold will be thick enough to support the plaster.
If you have only a small hole made by the wire or needle and thread, don't worry about sealing it--the alginate probably won't leak.
Since the alginate will set quickly, ensure the parts are flat, attached and ready to go before mixing alginate.
If your pig's feet have been cut in half (like mine), cast two. You can join them together to make a complete foot later.