Open document

Insert - Header

Click on page number

Put stuff in header

That's it...

Trying to work out what bit of that isn't working!

The way to cock it up is to insert a section break or have different first page or to have a document with different first and subsequent pages which you then try to make work right without adding a section break. If you show the fields and sections with ¶ then it might make it clearer where your problem lies