Lens flare

I am afraid there is no dedicated lens flare tutorial. The quickest way would probably be downloading some lens flare textures for example from
http://thinkdesignblog.com/free-textures-15-high-res-lens-flare-textures.htm then opening a photo, creating a new layer and copy/pasting the lens flare texture into the new layer (in Firefox, you can Copy the image directly), rescaling it (for example by right-clicking on the pasted floating selection and selecting "Fit selection to window") and setting the blending mode of the new layer to "Screen" or "Add".