This is a pretty straightforward tutorial.
1) Press D and then X to change your foreground color to white and your background color to black.
2) File>New. Use preset 640x480 with Background color as the background contents.
3) Create a new group, then a new layer.
4) Rename the layer "Core."
5) Press Control+Backspace to fill this new layer with black.
6) Change the blending mode of "Core" to screen
7) Press P to bring out the pen tool and use these settings
8) Start clicking and making anchor points to create a shape like a lightsaber. (Note: It does not have to be round, but it looks better round :))
9) Press control+e to merge layers
10) Filter>Blur>Gaussian Blur
11) Duplicate Layer(x2)
12) On Core Copy 2; - Filter>Blur>Gaussian Blur - 30px
13) Control+J, Control+E
14) On Core Copy; - Filter>Blur>Gaussian Blur - 5px
15) Select Core Copy 2; Press control+E two times.
17) The fine ------ coarse slider should be set in the middle; midtones selected.
18) Click on a color and click OK.
19) Control+U - Colorize unchecked +100 saturation (less if you want).
20) Right click any layer > flatten image
Now you are done!
here is my result: