Window and a reflector card, then should probably do it, depending of course on format, subject distance, and choice of film.
The principles are the same with natural light as with studio light, but you can't move the sun, so you have to observe the light and see when it's best for the look you are after (direct? bright but indirect? overcast?), and then you need to orient the subject for the effect you want. The geometry is the same as with artificial light, but the sun is now the key light, and your reflector is the fill.