Here are the steps users must do in SAM once GST-registered:
- Ensure GST Registered checkbox is ticked, and update the fields in this section

- Open "Document Setting Import" module and click on "Get Document".
Once the list is populated, click "Update".
This step updates your document's layouts to show "x% GST" amount where applicable.
- Open "Default Setting Registration" module and navigate to Invoice tab.
Click "Edit" -> change the document title to "Tax Invoice" -> click "Save"

For more information on GST functions in SAM, refer to this section of the Support Knowledgebase.