SMARTFORMS

I have used a smartform 'ZMMPO_A', a copy of standard smartform '/SMB40/MMPO_A' for printing a purchase order in MM module. The form works fine.

Now I want to display the various taxes(CST, VAT etc..) in the Main window for each item. The form should display the base values of each item, tax amount for each item and the grand total will be the sum of all these values.Please give me a step by step guideline how to achieve this.