My second attempt worked out sooo much better than the first. It came right off the mold and didn't stick to anything! I took it off after 24 hours and roughed up the edges a little, and ran a wire brush over it. Much easier. Using plastic bags around the form and underneath - perfect.
A few more details - try to make it look as much like stone as possible. Now, just wait a month for it to set.
You have got to try this...
Get your hands dirty!