The Orton image has traditionally been done using slide film with the first, sharp, image overexposed by two stops and the second, out of focus image, over exposed by one stop. It is important to use a tripod for this type of work to ensure your photographic elements remain in register on the film. The shots were then sandwiched together in single slide mount to produce a beautiful, impressionistic image. We are beyond that now and with so many things in the world of digital photography, we can now duplicate the same effect in multiple ways.
For purists, you can use the same technique as with film, two images, overexposed the same way, and in your image editing program place your out of focus image on tip of your in focus image as a layer and choose multiply as your blending mode. Of course this means always taking at least two images of your scene and always using a tripod. Another method is to use a single image a create a layer, now opening up the effect to any image you have in your collection.