Hi there - happy to explain what I did in Procreate. I loaded an image of the model (on a black background) as a layer in Procreate. I then load in a texture image on a layer above and set the blend mode of that layer to overlay. The texture then only shows up on the model and not on the background and all of the shading from the model shows through. There are some other blend modes which sometimes also work well - just have to experiment. With things like wood grain I then rotate, move, shear the texture image to line it up properly.
Perhaps I should also explain that to render the image of the model I output the stl file from shapr3d to dropbox and then download it back to my ipad and into i3D Viewer app - produces great renders
Hope this is helpful - let me know how you get on.