Help with pre-printed letterhead paper

Click the Page Layout tab on the ribbon.

At the left end, click the Margins button. If the Normal item (1" for all four margins) isn't selected, click it. This is the value that will be used for all pages except the first one.

Next, double-click in the header area, which opens the Header & Footer Tools tab on the ribbon. On that tab, check the box for Different First Page. Notice that the tag at the bottom of the header area now says "First Page Header". Leave the cursor in the header paragraph.

Again click the Page Layout tab. In the Paragraph group, under Spacing, change the After value to 132 pt.  To arrive at this number, I started with the desired 2.5", subtracted the 0.5" default distance from the top edge of the paper to the top of the header text, multiplied by 72 pt per inch, and subtracted the 12 pt depth of the header text paragraph:

   ((2.5 - 0.5) * 72) - 12 = 132

Move the cursor to the First Page Footer. On the Page Layout tab, in the Paragraph group, under Spacing, change the Before value to 60 pt. (The math is the same, but starting with 1.5".)

Finally, double-click in the document body to close the header and footer areas, and save.