Making a Prism

Open up your sparkle tube that you saved at the start of this tutorial (File, Open, navigate to where the tube is, then hit "OK")

Take a copy of the tube (Edit, Copy)

Go back to your octagon image

Paste (Edit, Paste as new layer) the sparkle onto each of the corners where the red, green and blue light comes out (it may be more comfortable and more accurate if you zoom in on your image to do this bit)

Merge the layers (Layers, Merge, Merge Visible)

Take a copy of this image (Edit, Copy) 

Go to your image with the black background

Paste as new layer (Edit, Paste as new layer) 

Adjust octagon so that everything lines up and you can't see the joins 

Merge your layers again (Layers, Merge, Merge Visible)

Crop of any wasted space

Save your work

You're finished!!

Now that you know the basics of how to make it, you can, of course, make any changes or improvements you think it needs :o)