We're going to need a good-sized lump of modelling clay ... to take a lump of clay and cut parts away then that would be called "subtractive". Give it lots of time to dry. More clay takes longer ...

All you need for this project are air-dry clay, a rolling pin ... the U.S., you can get a 2.2-lb pack of DAS Air-Hardening Modeling Clay at Michael's craft stores for $11.99.