Well… the idea would be you could take a photo of an existing pattern piece… it then needs to be resized to the real size. IE: you would include for example a 4inch x 4inch square in the photo, and then that square needs to be resized to 4x4 on the Seamly draft screen. Then from there you can trace over the piece, and it should be in real size.
If you know what the size of the pattern you traced, you can use formulas to make the pattern to size. I outlined a method with my shift dress pattern: