Hi,
I would recommend you to go through the Drawing Flattener help file for detailed study. However, you may start by using following steps:
1. Load 'Bentley AutoPLANT Drawing Flattener'.
2. Apply 'File > Add Files to List...' menu command.
3. Pick appropriate option from the 'Choose the browsing mode' and select the drawing you want to flatten.
The drawing will appear in the Drawing Flattener interface.
4. Press the 'Start Flattening' tool-button.
The drawing will be flattened and the flattened drawing can be opened from 'File > Open Destination'.