SHD0 modification

Dear Friends,

I am currently trying to understand how does the SHD0 tcode exactly work. I've checked many recourses available on internet and tried to implement it in SAP system following step by step tutorials, but unfortunately something is going wrong and I can't make it work. Here is what I need:

I need to make some modifications and assign some users so they can't make any inputs in "Work", "Number", and "Normal Duration" in Operations tab in SAP PM module (tcode IW32)

I would really appreciate if you can give me some directions.

Thanks!