The first stage is to make sure the right area is selected
I want to make all the photograph, apart from the face and centre part of the hair soft. Choose the Oval Marquee tool from the tool bar and click top left then drag down to the bottom right where you want the oval to extend to.
If you applied the blur now it would be the face that changes so we need to invert the selection. In Photoshop this is found in the Select Menu. Select>Inverse. Now the selection changes so that all the unselected area is selected (indicated by the marching ant - dotted lines going all around the fram as well as the middle oval that we originally had.
If we now apply the blur the correct area will be affected but the edge where the oval is will be apparent. To avoid this we need to feather the new selection Select>Feather. My photo is a 17Mb file so I needed a large feather for it to have any affect. I went for the maximum 250 pixels which will give a very gradual effect and blend subtly into the facial areas.