I used the recipe from A-M whipped soap tutorial. I added 55g Liquid Castille Soap to 225g M & P soap after melting, fragrancing and colouring. Then I used an electric whisk (not a beater) and whipped it mercilessly until it was light and holding peaks (like meringue). Then into a plastic piping bag with a decorative nozzle and then off I went! I just kept topping up the bag as it emptied and about halfway through the whipped soap was going a bit hard (in the bag and the bowl) so I reheated it for about 15 seconds and then re-whipped.
Three things I think are crucial:
1) Colour it dark - as it whips it lightens incredibly
2) Work FAST!! There's nothing worse than fiddling around and discovering that the soap in the bag has gone rock hard and you simply CANNOT squeeze it out of the nozzle!!
3) If it looks horrible - scrape it off and start again!! I absolutely LOVED that part!
Next time I'll pipe the frosting even higher!