KM33 ALL IN ONE

Works best with photos with lights at the background:

1.) Open photo in PS.

2.) Create a copy (B/G copy or main photo copy or "Layer 1")

3.) Adjust levels of "Layer 1" and make it brighter or lighter, don't focus much on the model/subject. Focus on the B/G i.e. Bokeh, light trails, etc.

4.) Paint the model/subject black using brush. MAKE SURE that your model/subject is all blacked out. You can also paint everything that you dont want to be affected by the diffussion or flare

5.) Make a copy of "Layer 1". You should now see 3 Layers. Namely:
"Background"(Your main image),
"Layer 1"(The lighter and painted one),
"Layer 1 copy"
*You can rename this to your liking, if you're O.C. :P

6.) Go to "Layer 1 copy" Change blending mode to screen.

7.) (Working on Layer 1 copy) Go to Filter>Blur>Motion Blur
*Change angle to 45degrees
*Change distance to your liking. This will direct the size of the flare.
In this particular photo of Aiko, I set distance to 800
You should now see a slight diffusion.

8.) If you're not happy with the brightness of the diffussion since the last filter that you used is "Motion Blur" you can just press Ctrl F(Windows) to have it running again, and you'll have a brighter diffusion (Optional). You can always adjust the opacity of the layer if you think it's too much

9.) Go back to "Layer 1" and make another copy... Drag it on top of the layers. You should now have 4 Layers. The latest 1 is "Layer 1 copy 2", DONT Forget to change blending mode to Screen.

10.) (Working on "Layer 1 copy 2") Go to Filter>Blur>Motion Blur
*Change angle to "-45degrees (Negative 45)"
*Change distance to your liking. This will direct the size of the flare.
As I've said, in this particular photo of Aiko, I set distance to 800

11.) and again, If you're not happy with the brightness of the diffussion since the last filter that you used is "Motion Blur" you can just press Ctrl F to have it running again, and you'll have a brighter diffusion (Optional). You can always adjust the opacity of the layer if you think it's too much.

 You should now see a STAR LIKE diffussion and the model/subject is blacked out.

12.) Now, go to the top most layer which is "Layer 1 copy 2", anyway, if you've followed everything... highlight in the layer pannel is on the top most. Now press "Ctrl E" (Windows) TO MERGE Layer 1 copy 2 and Layer 1 copy.
* Upon pressing Ctrl E you should now see only 3 layers namely: "Layer 1 copy", "Layer 1" and "Background".
Still you can only see the star like diffussion from the bokeh and the blacked out subject.

13.) Go to "Layer 1" and untick the "eye" in the layers pannel. It'll HIDE Layer 1.

And viola!!! Subject is there and you got your effect.
Extra: if you have not cleanly done the "blacking out", you can always mask the extras.(You wont be needing this if you have perfectly done step 4.)
*Go to Layer 1 copy and click on "mask" Gray rectangle with white circle in the middle in the layers pannel [O] <--- Somehow looking like this
*Click on the mask on Layer 1 copy (White rectangular box) and just paint the part that you dont want to be affected with black using brush.

Disclaimer: I don't claim full credit for this effect. I've only seen this somewhere. I just made it more accurate "by painting the model/subject black" rather than playing with the Treshold and I added, "adding masking" at the end just in case you have not perfected step 4 so you'll have a cleaner image. Made everything simplier/adjustable and a lot more understandable :)