I would use a mini spatula or the end of any decorating tool that is flat and about 1/4" wide. Put a thick layer of BC on. Start turning your cake on the turntable and hold the spatula (or whatever) against the cake, digging it in a little - as deep as you like. You can either spiral up as you turn or just stop and start each line to make them straight. As you drag the tool, excess BC will build up and kind of flop over, like in the photo.