Fold the impression (cut or uncut) into several folds according to page number and sequence. The fold with multi-pages is called a signature of book-block. After collocating the folded signatures into volumes by order and combine them, it is called saddle stitching.
Saddle stitching is when a project with a page count divisible by 4 is laid flat on the saddle. Staples are usually placed inside the spine (the crease in this case), also known as the gutter, to hold the pages together. Often times a saddle stitched booklet only requires 2 staples. This technique is not to be confused with side stitching or side staple where the staples are placed a ¼" from the spine on the front of the piece as opposed to inside the spine, which prevents the project from opening flat as it would with saddle stitching. Saddle Stitching is best used with smaller page counts because as the page count goes up so does the amount of creeping which can occur.
Creeping is when the inside pages "creep" out beyond the outside pages. This can be fixed by trimming the inner pages. However, this can result in ruined margins or cut off text. Trimming can be accounted for by the designer making the inside pages slightly smaller to accommodate the trimming, but this can be incredibly time consuming (and costly). The best way to avoid creeping is to leave saddle stitching to smaller projects such as low page count magazines, calendars, zines, comic books or small pamphlets.