What size will your book be? (This is called "trim size".)

I usually do 5.5"x8.5".

The most popular vendor is Amazon KDP, but you might also want to use Ingram or perhaps Lulu to distribute your books. You may find some trim sizes unique to them at their sites. Once you've decided, go to...

Paper Layout > Print Size > More Paper Sizes

Under Paper tab, set trim size, don’t click OK

These instructions are for up to 500 pages. Here's what to put:

Margins > Top: .76", Bottom: .76", Inside: .76”

Outside: .6”

Gutter: .14”

Choose Mirror Margins

Apply it to the Whole Document

Do not click OK.

If you have more than 500 pages, increase your margins according to this guide.

Next, on to Layout.

For section start, choose New Page.

Check both
Different Odd and Even
Different first page

Enter 0.3 for both Header and Footer--but you can adjust this for personal taste.

Leave the page alignment at Top.

NOW you can click OK