For cutable materials (fabric, linoleum, sheet metal):

Get the right cutter for the job. Metal or plant shears can work for many materials. Scissors may be fine for fabric. A sharp knife may work for linoleum (cut on top of wood board). Be careful of the sharp edges when cutting sheet metal. Make a template of the shape you need out of newspaper then draw it on your material and cut it out.

For moldable materials (Model Magic, plaster):

Create a mold out of a carveable material (eg. styrofoam, plastiscene) or a formable material (eg. aluminium foil). Then strech out the model magic and put it on the mold (secure it in place with toothpicks if needed). For plaster simply cover the mold. Watch out for material flow - it's best to build it in stages if it is very rounded or all the material will flow downhill and the top parts won't have much left. You can leave toothpicks sticking out of previous sections so the next section has something to hold on to.

For carveable materials (neoprene, styrofoam, balsa wood):

Cut with appropriate knife (exacto blades are great) or burn/melt into shape with soldering iron (caution - can be very smelly depending on the material - do outdoors or with lots of ventialation).

For heavier materials (thick wood):

Cut with appropriate tools (jigsaw, saw) and shape (dremel, file) then surface (sandpaper) and paint.