You can buy a special turntable with spikes for decorating dummies , or you can be cheap like I am and make your own.
Take a cake drum, a small size is okay.
|Duct tape fixes everything.|
|It's not going anywhere!|
Get rid of the nails after the cake is decorated or you might impale yourself by accident later. If you use the board that the cake will be displayed on to decorate it, though, you can just leave it in place if you attach the dummy to the board with epoxy when you do the impaling, then remove the nails from the bottom of the board later.