Tested on Microsoft Office Word 2007 for Windows. I expect most recent versions of Microsoft Word will behave pretty much the same way, except the shortcut keys may be different. If the below doesn’t work, check your Help documentation for instructions on how to create fields. You should be able to insert a page number by typing the word “page” and then doing something. Once you figure out what that “something” is, you should be able to use it for the below.
Click at the beginning of your second section (or at least somewhere on the first page of the second section) and type “set sec2start page”. (“sec2start” is just an arbitrary name I have chosen – you can use something different – but, as you might guess, “set” and “page” are keywords.) Select the word “page” and press Ctrl+F9. Curly braces will appear around the word “page”; as in “set sec2start { page }”. Now select the entire phrase and press Ctrl+F9 again. More curly braces will appear: “{ set sec2start { page } }”. Now press F9.
Now go into the header and type “= page - ref sec2start +1”. As before, select the word “page” and press Ctrl+F9. Then select “ref sec2start” and press Ctrl+F9. As before, curly braces will appear: “= { page } - { ref sec2start } +1”. Select this entire phrase and press Ctrl+F9 one more time. And press F9 again.
Finally, press Alt+F9.
You should now have a magic number in the header of Section 2 that is the page number within Section 2 – specifically, relative to the sec2start bookmark.
Repeat for each section.