See these links:
A Short Tutorial on User Exits
Point3 is what you need to learn in the simplest form. Tcode SMOD has been not necessary here, which often figure during talks about user-exits. In point3 when it was told double click on include ZXVVAU04 it is SMOD. Measn after activating CMOD project, you can go to SMOD to write your code in the corresponding include.
Means in the present case you create a project throguh CMOD tcode say ZPM_IPRM, and in that project under enhancements button you specify present exit namely IPRM0004. Save, come back to initial screen and Activate the project. .
Now run tcode SMOD, click on Test and then double click on EXIT_SAPLIWP3_004 > D'click on click on include name ZXPRMU06 . If it is not already handled system will take your consent to create it. Create it and there you reach the place to put your code which does all your requirements. I
Means if I could provide you some code, you need to put it there.
Note that all this exercise needs to be done in DEV210 (ABAP client) . While doing so a TrReq will be generated (like customizing) which you need to transport to Quality and finally Production. But before transports the effects due to this implementation will be seen in DEV100 and DEV220. So you can test there. Any corrections you make accordingly in DEV210 will be updating the objects in same TrReq (until it is released for transport)