I do them just like Jodie - the only recommendation to newbies trying that technique is not to cut too far into the center. Cutting too far in and then melting back out doesn't work out so hot. Also if you want free-form sculpture I'd cold seal the bottom first. When I do them I also coat a clear gather in color to save money because I'm dirt poor, but this also gives a cool effect to the finished product depending on my method of coating...
I have recently been putting them on bottle stoppers, so I leave a tail coming out of the bottom so that the glue holds. If not that'd be where the cold seal would go, and hopefully wouldn't get melted off partway through! Good luck