You now have a choice for how to create custom pages. If you want to select photos and memories and have the pages automatically created, you can use the auto build process. If you want to create the custom pages from scratch, you can use advanced editing.
To auto build the pages or manually create the pages log in and select Build Custom Pages.
Step 1 when using the auto build process is to select photos from your photo sources to add to the selection tray. After you add the photos to the tray, Step 2 (select Next) is adding Memories. You will then build the pages which means that all of your photos and memories will automatically be added to custom pages. These pages will be editable if you decide that you want to make changes to the pages. Here is an article showing how to auto build your custom pages:
Using advanced editing means that you will manually add photos, memories, and other content to the pages. Select Try the Page Editor to skip the auto build steps.