Hi jingying Sony,
First, you need to check whether you are using classic WHT or EWT. Actually the setting is quite different between the two.
Second, generally the setting processing is as followed,
1) Determine on which step system should calculate the WHT, invoice or payment.
2) Create a WHT code and type
3) Assign code and type to company code
4) In vendor master data, input the relevant code and type by flagged "liable".
5) Set WHT posting account
Hope it would be helpful.