The edges of pictures are duplicated and extended in the printing process. That's how it continues over the sides so no frame is needed. A 2-sided (arrow) mark is also printed to help with aligning it during the stretching process
They are not all easy to find . If you can only find ONE then while the print is FACE UP on the table gently place the completed stretcher bar frame (like step 4) on top to find the other corners
Put your finger in the corner mark. Reach around back to mark the spot. NOW put 3-4 staples or stick pins somewhere (u-will remove them later). This will keep the picture in place when you NOW flip it over to staple it on the stretcher bars. Use stainless steel or you'll have rust later.