I remember seeing this type of shot for the first time in the late 90s when I was editing Digital PhotoFX magazine and have been to have a go myself, so I positioned some shoes on a cloth background. photographed them and then took a second shot switching the boots for my feet so they'd be in the same position when I come to merge in Photoshop. I then used two layers and masks to blend the two, then added a texture layer and blended that.