Built from an sacan of my own feet, as Gyrobot describes.
I produced the . stl file in a simpler way: i cutted a template with the contour of each of my feet from A4 paper sheets, to avoid installing new software and learning it in ordee to cut the shape virtually in the file. Then I imported the . jpeg with Cura, which I use for slicing, and has a simple way to transform the shades of gray into heights. This worked quite well.
An advice: control the size of the scan (not too many pixels, not too many shades of gray) as the . stl may become slow to slice and print. A lot of detail is really not needed.
I printed slowly (33mm/s) and low res (0.35 layers, 22% infill, no top or down layers) and the print took about 3 hrs.