How to assign user defined smartform to standard transactions?

Hi,

I have developed two smartforms for PO and domestic Invoice.

For both i have written driver program (zprogram).

I am getting output very well by running these driver programs.

I want to assign this smartforms to standard transactions (ME21N & VF03) so that i will get output for the

runtime value in the screen.

Please do reply if have any clue.