The benefits of having a scientist husband, beware this is very fun!… but expensive but saves so much mass going to the landfill. The bowl below is the final product of 1L of acetone bought from Bunnings Warehouse for $15, some wood shavings for bulk and an entire bag of polystyrene from all of the packings we got stocking up our new house. Its a very simple process, simply fill a glass bowl with acetone (make sure you are outside in a well ventilated area the fumes can be quite bad), and start adding polystyrene until you can't add any more. It will seem to boil, thats just the air in the polystyrene escaping. Eventually you will get a puddle of goo ready to form into a shape. We find that it is easier when you mix the polystyrene in with something to bulk out the solution such as sawdust, flour or sand and helps it form a shape.
Remember to follow safety instructions on the bottle: Do this in a well ventilated area (outside preferably) and do not touch the acetone and wear gloves (the acetone dries your skin out really quickly)