this is not easy. converting a figure is some what easier but still no easy task.

you will need milliput or some other medium a selection of wax carver tools to help you along.

over on Scratch mods forum there will be a Sculpting GB soon ish.. held by one of the fantastic Sculptors over there, Paul Philips, I recomend you keep an eye on that, you may need to register to see it but it will be worth it

http://modelerssocialclub.proboards.com/board/15/figures-gb