Making something glow is relatively easy. I tend to use layers a lot so I'll reference doing things to the layer, but you can use selections as well.
- First, duplicate the layer with the item you want to glow and rename the layer for reference. I usually keep the same layer name appending the word "glow small".
- Move the new layer just below the original one.
- Now repeat the following steps for each color that is being changed.
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- Hide the original layer so we can see what we're doing.
- Find the color that you want to use, make it darker and set it as the foreground color. For example, using HTML codes of #F00 for a red neon look, I'd choose #C00 for the glow color.
- Select everything in the new "glow" layer that should glow with a particular color. I usually use the fuzzy select (U) and the shift key to get everything.
- Grow the selection by 2 pixels or so. (Select -> Grow)
- Fill the selected area with the FG color (Edit -> Fill with FG color or Ctrl-+)
- Duplicate this layer as well, moving the new layer below, preferably calling it "glow large"
- Clear the selection (Select -> None)
- Apply a Guassian blur of about 8 pixels to the "glow small" layer. (Filter -> Blur -> Guassian Blur). Note, if you hide the "glow large" layer as well temporarily, you'll see the initial glowing effect.
- Apply a Guassian blue of about 18 pixels to the "glow large" layer. (Filter -> Blur -> Guassian Blur).
- Show all three layers and you should have a nice effect going on.
Adjust the pixel amounts to tweak the effect to your personal preferences, but those should give a reasonable view.
This effect looks really nice with outlines as well. Basically use the Select -> Border menu option to get an outine on the initial layer. I'll have a detailed tip up for that soon.