How it works / How to do it manually

Here's a basic rundown of what the Script-Fu does, and how to do basically the same thing manually (there are slight differences).  The manual process should basically be translatable to any layer-base graphics program (eg. Photo$hop)

OK, once there is an active selection, the blade-roto script-fu stores this selection for recalling several times during the running of the script.  But doing this manually is a pain, so there are some differences between the way the script-fu works and how you (well, how I) would do it manually.

On a new transparent layer, the selection is then slightly feathered - with the script-fu, this is 1/10th of the "FX Size" value, manually you'd use around 4 - 5 pixels for a mid-sized image, more for bigger pics.   This is the "blade" layer, and gives a slightly "fuzzy" look to the white blade.  Fill the selection with white, and then fill it again - this "doubles-up" the whiteness, making it appear solid, whilst maintaining a fuzzy edge.  The script-fu also does this.

Now, this is where the manual method and the script-fu method differ - making the selection that is the basis of the three corona layers - the tight corona behind the blade, the loose corona "glow" behind the blade, and the tight corona colour-emphasis layer above the blade.
Manually, I select/none, clearing any selection, then use the "magic wand" (fuzzy selection), set at it's maximum, and click on the "blade" - this gives a selection as big as the outer-most perimeter of the fuzzy blade edge.  For good measure, I then grow the selection (select/grow) by 1 or 2 pixels (more for bigger images) to give a nice defined corona selection.  The script-fu simply recalls the stored "original" selection, and grows the selection by the defined "Grow Size" to give an approximation of the size the selection would be if you did it the manual way.

Both the manual way and script-fu now have a selection that is essentially a grown selection of the blade - it should be around 2 to 3 times the width of the blade.  On a new transparent layer, this selection is feathered by half of the "FX Size" - about 20 to 30 pixels for a mid-sized image.  This selection is then filled with the corona colour - it may look a bit dark, but that is fixed up later.  Of course, this layer is now "above" the blade layer - simply lower the layer so that it sits between the original picture and the blade layer.  Then Select/None.
Next we duplicate this "corona layer", and raise it so it sits "above" the blade layer - this becomes our "corona emphasis" layer.  In the Layer dialogue box, change the "Mode" of this layer from "normal" to "Color" - this emphasises the colour beneath.  It may be a little extreme, so reduce the opacity of the layer to 80%

Now we make a new transparent layer below the corona - this will be our "corona glow" layer.   Manually, you will have to go back and repeat the process of selecting the fuzzy blade and growing the selection - the script-fu simply loads back in the original selection and grows it again.  We then apply some feathering - the "FX Size" value in the case of the script-fu, or twice the value you feathered the corona layer.  Fill this selection with the corona colour, then in the Layer dialogue box reduce the opacity to 50%.

We then go back and lighten the colours of the "corona" and "corona glow" layers ("corona emphasis" is left alone).  Simply select each layer, and use Image/Colours/Hue-Saturation, setting "Lightness" to 100%.  This makes the corona and corona glow look more like they are actually emitted light, but on very light backgrounds, such as deserts, you may want to use less lightness.

Now for the emitter flare.  Turn off the visibility of all layers except "corona" and "blade", then merge visible layers - this will combine the blade and corona layers into one layer.   Run Filters/Light Effects/Flare FX and select the coordinates where the emitter flare should be.

Make all layers visible again, and that is essentially the whole process of the script-fu.