As mentioned above, a mini scrapbook can be an actual album, or it can be used to add something unexpected to an older page.
The first step you need to decide on is the purpose of your scrapbook mini book.
If it's going to be a standalone album, you can create the album or purchase a small photo album (the type that holds one photo per sleeve).
Small photo albums can be purchased at a retail store, a craft store or even a dollar store.
If you are thinking about using a mini scrapbook to enhance a 12×12 scrapbook page or even a handmade greeting card, create an album of six front/back pages or less.
Usually when a mini scrapbook is used in this matter, the purpose of the scrapbook is to provide more details of the scrapbook page theme.
For example, let's say your scrapbooking layout theme is about your child enjoying his birthday cake as he turns two. The mini scrapbook might contain photos of your daughter helping to make the cake, the two of you decorating the cake, your son running his finger through the frosting, etc.