If you don't have a PDF of your fixed layout book upfront but if you have the single pages (as images) that make up the book, you need to create the book from scratch clicking on "create new book" on your StreetLib Write account's homepage and choosing the right editor among "basic Fixed Layout book", "Comic Book" or "Kids book" (find out more about book types here).
Once your book's title inserted and the right book type chosen, click on "create". The editor where you'll be able to import your pages will open.
To do so, click on "Add" on the left window.
There upload the image constituing the first page of your book and click on "create page".
Proceed with the following pages, with the same technique. You can add titles to each pages. (They will create the Table of Content.)
You can change the pages' order whenever you want, with a simple drag and drop on the left window of the editor where all your pages are listed one after the other. (left click on the page you want to move, hold that click and move your mouth to the spot you want it)