Drag it into place, or type exact coordinates
Two ways to position, because the two jobs are different. For a one-off, drag it where it looks right. For something that has to land in the same spot on every document you produce, type the numbers.
- Drag to move, corner handles to resize, a handle above to rotate
- Double-click the text on the page to edit it in place
- Position is a percentage of the page, so 50 / 50 is the centre of every page — including documents that mix page sizes
That last point is worth a sentence of its own: because position is relative to the page rather than measured in absolute points, the same setting lands in the same visual place on a letter page and an A4 page in the same file.