Before starting the layout, check the measurements for the format you have selected. The files should be created exactly in these measurements, as they all include a safety margin of approximately 0.5 cm(Fineart = 0.5 cm) around the spread. The templates available for download include guides that demarcate this area.
Within the help menu, on the Downloads page, templates (in .psd format) for core layouts are available for download as well as masks for creating images for all customisable accessories. It is of utmost importance to respect both so that the printing of your product comes out as desired.
You can download any template or mask here