The link above will direct you to a very resourceful page teaching you the basics of using a screwdriver. HOWEVER, when using a screwdriver, or more specifically, the flat head screwdriver for plaster, it will defy everything in that link. You will use the screwdriver simply as a tool to prop the negative cast open.
During your study period at GBC, you will come across a cast that needs to have a plaster buildup around the proximal brims; and sometimes you may make this buildup much higher than necessary. Hacksaw will be your best friend for the removal of the excess plaster. Simply draw a line across the top of the cast where you want to remove, and using the hacksaw to slowly cut your way through the plaster. Then using a flathead screwdriver, start chipping away the excess plaster.