ok it simple..

Fill with the Background in Black..
Create a new layer..(Fill with watever color Beside Black)
using ur polygonal lasso tool..
to cut a Triangle shape.in the top right of the corner..after
Edit->Cut
Create a new layer again..
Ctrl+V to ur new layer (Notice:at tis moment..u see nothing)

To ur Blending Mode~>
Set OuterGlow
Blend mode:Normal
Color:Black
Opacity:85
Spread: 7
Size:6
(u can play with ur own,if u dont inssit to follow)

after here come out with another Triangle layout,at the previos wat u did..
here go Edit~>Transform~>Rotate 180

here u know wat to do..just move to the other side..
here come my result!






Attached thumbnail(s)

doing manually..is a best ways u can improved ur photoshop skills..

keep it on!

but sometimes..u need to use some plug in for some nice effect adjustment.