1. First, start CASS software to ensure that topographic map data has been loaded and ready. Select "Import" or "Open" option in CASS, browse and select the topographic map file to be cut.

2. Secondly, determine the range of cutting topographic map, and select "cutting" or "cutting" option in CASS software.

3. Finally, cut the topographic map according to the range of the drawing frame, and select the "Save" option to save the cut topographic map to the specified file path.