The only problem is that even the best flashguns from Canon, Nikon and the like only have a small light source which causes hard shadows. Softer shadows need a large light source.
You can purchase any number of light modifiers that can be attached to the front of your flashguns flash tube to make its footprint larger and therefore give you softer shadows. However, here is a cheeky hack that can give you surprisingly good results and it begins with a plastic sandwich container/lunchbox.
The sandwich box needs to be translucent, rather than clear, so it helps to diffuse the light better. You can also use the kind of translucent plastic containers you often get from your local takeaway restaurant. You’ll need to trace the outline of your flash tube along one side of the box and use a craft knife to cut out an aperture that the flashgun can slide into snugly and secure it with a little duct tape.
Pop the lid off the container and line the bottom of it with silver foil. The idea here is that you want to reflect as much light forward as possible and not lose too much out of the back and possibly the top as well. The flashgun then slides into that aperture and, when it fires, it illuminates the container and the silver foil turning it into a small softbox.
Because the container is considerably larger in size than the flash tube, shadows will be softer as a result. You will, of course, lose some of your flash’s output power since it is being diffused and spread out, but an extra bit of flash exposure compensation should make up for the shortfall. It may not look pretty, but it works. By the way, you can even make a miniature version out of a translucent 35mm film canister that can sit atop your camera’s pop-up flash.