How to obtain the file offset from an RVA???

Hello,

I'm writing a bit of code to better understand PE format, but one thing I still can't get is how a file offset (position in the physical file on disk) is obtained from an RVA. I don't need a tool, I'd need to understand. Can anyone explain (or provide the function) it in terms of the members of the C structures used to read the DOS, COFF, PE, Section headers???
Thx.

yaa