How do I turn my single-face document into facing pages?

@Gerben – to get  predictable results, I recommend working with two files.
Be forewarned, It's a complex process:

If your basic single sided document starts with an odd page number:

1. The basics:

docS = your single sided document with single sided master pages

docDD = a all new double sided document you have to create

     with: 1 double sided master page

               with:  page size: docDD page size

               with:  margins inside = docS  margins left

               with: margins outside = docS  margins right

               with: margins top = docS margins top

               with: margins bottom = docS margins bottom

     with: two pages

     with: start page number: 1

     with: baseline grid = docS baseline grid

     with: paragraph style definition of "Basic Paragraph Style" = docS "Basic Paragraph Style"

2. To transfer all pages from docS => docDD:

Select page 1 of docS in the pages panel

Choose "Move Pages..." in the flyout menu of your pages panel

     "Move Pages:" 1-n (where 1 is the first page name and "n" is the last page name of  docS)

     "Destinaion:" After Page 2

     "Move to:" docDD

Hit "OK"

3. All pages of docS are now trasferred to docDD

That the spread view in the pages panel is showing a somehow confusing picture of single pages spreads after page 2 should not bother us too much :-)

From the pages panel you see something like this:
1, 2, [3],[4],[5],[6] …

4. Now the "hard" part, where we have to be very cautious not to get wrong page contents positions and a wrong stacking order of page elements:

4.1 Select all pages after page 2 in the pages panel and check "Allow Selected Spread to Shuffle"

4.2 Select ALL spreads in the pages panel and un-check "Allow Selected Spreads to Shuffle":

Result should be: [1], [2-3],[4-5], etc...

4.3 Select ALL spreads in the pages panel and un-check "Allow Document Pages to Shuffle"

Now:

4.4 Select page 1 and delete page 1

4.5 Select page 2 and delete page 2

Further:

4.6 Select ALL spreads in the pages panel and check "Allow Document Pages to Shuffle"

4.7 Select ALL spreads in the pages panel and check "Allow Selected Spreads to Shuffle"

Now, we are basically done…

5. Then change the contents of master spread 1 of docDD as you wish and/or add master spreads as you wish.

Hope that helps,

Uwe

File>Document Setup

Click Facing Pages

Then in the master pages add a page beside the master page to make a spread.