First, we make the map. Start by desaturating your main source image. In this case, the banana. This gives use the shades of grey we were talking about earlier. However, if you remember, shades of grey don't have as big of an impact on an image as blacks and whites do. So, in order to get those blacks and whites we're going to adjust the brightness/contrast on this image to add contrast to what we have.
Now you probably notice that the different values are defined a bit too harsly where they meet. This will make our texture look choppier than it should (kind of like how the candid example looked above). Therefore we want to add a bit of blur to smooth this out. Typically one or two pixels of blur will get the job done.
What we have now is essentially our displacement map. Save the image you have now as a .PSD and remember where you put it. We will need it later.