First you have to pick a nice photo. Make sure you pick a photo in which you will not cut away vital parts (such as eyes) later on. Chose which part of the photo you want to use and cut out a square. Put the square in a new document.
On this new document you can:
Change image size to 30cmx30cm. Desaturate the image. Change brightness and contrast until you get the desired result. You might want use the paintbrush to get rid of unwanted details in the background. You might want to reverse the picture, otherwise you will end up (like me) with a mirror image of your original photo.