a Problem about how to design the soft interface of the Custom Instruction

In my project, I want to add the Custom Instruction.

I already find out the document of User Guild, But I still don't Understand the Soft Interface design.

Does anyone can help me??

and my project the custom component only use a 32 bits adder to try the method of custom instruction.

so, does anyone could help me?? or give me some examples??

Thank you very much.