I have used 2 methods over the years, one is to stretch a black stocking over the lens, either for the whole or part of the exposure, I tended to expose with stocking over the lens for the whole exposure, and the stocking must be black, you can vary the diffusion effect by the tightness of the stocking on the lens, the tighter it is streached the less the diffusion, also make your exposure testswith the stocking in place, the second, which is a genuine soft focus, is to opt for long exposure, 20 seconds at least, and for the first half of the exposure expose with the lens sharp and for the second half focus and de focus the lens, which works a treat,