When you choose mirror margins, the margins of the left page are a mirror image of those on the right page. That is, the inside margins are the same width, and the outside margins are the same width.
1. On the Page Layout tab, in the Page Setup group, click Margins.
2. Click Mirrored.
To change the margin widths, click Margins, click Custom Margins, and then, in the Inside and Outside boxes, enter the widths that you want.