The first thing you need is an avatar. Here's one of mine that should do the trick nicely.
All right, we've got that. Now, make a new layer, and draw in the lightsaber handle. It shouldn't be too terribly difficult, as it's just a few pixels. You probably won't be able to get a lot of detail in, though.
Okay, now the blade! Make another new layer just below the handle, and pick your blade color. It should be relatively bright - I picked a pretty purple, ala Mace Windu. Draw in a sharp, but not pixel-edged, line three or four pixels wide in that color. Duplicate that layer, hit Preserve Transparency in the layers palette, and paint-bucket the thing pure white. You'll see some colored edges from the layer below - that's not a problem.
Hit the colored layer (below the white layer) with a Gaussian Blur of about three pixels. It seems a bit anemic now, but it is glowing. To remedy the weak glow, duplicate the blurred layer a few times, then merge the duplicates.
Hmm... still doesn't seem quite bright enough to me for it to be a proper lightsaber. That's easily remedied. Set the colored layer to 'Screen,' or 'Linear Dodge' if your Photoshop is of a recent enough version. Now it looks like the blade is actually shining onto your avatar, but what's this? It's invisible wherever the background is white!
To fix the above, duplicate your colored layer yet again and set it back to Normal mode. Move the Normal-mode layer below EVERYTHING except the white background - behind your avatar and all. There - now you can see it!
And there you have it, folks - one lightsaber!
