Open a new document in photoshop, 400x400 with a black background.
Create a new layer (Layer->New->Layer...) and fill it with black like the background.
Then, with the new layer selected, Filter->Render-Lens Flare... And do like I did here:
Now, scale it down like this:
by using the free transform tool (Ctrl + T or Edit->Free Transform)
When that's done, duplicate the layer you're vorking on (Layer->Duplicate Layer...) and rotate the copy 90 degrees (by using the free transform tool again) like this:
Change the new layers blend mode to "Lighten":
And merge the two layers together by Layer->Merge Down or simply Ctrl + E
Duplicate the merged layer, set the blend mode to "Lighten" again and rotate it 45 degrees like this:
And the two final steps:
And some blending options:
And that's about it.