Just got this from Apple Support for Pages v6.05 and Sierra.
If you're wanting a text letterhead template, as I did, open a blank document, go to Document in the Format menu and unclick header. Create a text box, fill it with your preferred letterhead in font and layout as you will, and move it to where you want it (presumably at least a half inch from the top of the document if you intend to print the result).
I also want my template to have today's date, a name/address line (single-spaced), and a "Dear " line with the correct line spacing. When I typed those in, they were floating alongside my right-aligned text box, so I had to use returns to push them below the letterhead.
Once I had everything formatted as and where I wanted it, I Saved As Template... Voila, the letterhead is not a header and therefore not repeating on following pages.
Presumably, if you want an image for the first page header, do as above but insert an image, not a text box.
If you don't want a template, do everything as above and save it as a document. Easy. No messing around with Sections...