If you'd like to send a client a soft proof on a book you've designed in our on-line editor, It's really simple.
First, Log in on the professional site.
Browse to the project in "my projects" and click the "share" button.
You'll see lots of options to share your project. It's probably simplest just to use the email option.
This will create an email with the URL to the album in it. You may edit the body of the email to explain that this is a preview of the album and that they should get back to you with any changes or an "OK" to proceed with the printing.
Send them the email and hope for the best :D