Don't worry Martin, Layer won't bite.
Like much of this, just play around a bit. No need to take notes. Push buttons and turn knobs until something breaks.
Ivo's steps are pretty complete I think, so you can just follow them along. Though I would process the NB to taste in the normal fashion rather than trying to match up with visual.
Now, what does everything mean and how does it work? Layer is a triple window module: Background, Foreground, and Combined (your result), in that order. I think. Actually I always forget, even though which is which flashes on the screen upon module entry. But no worries, there are all sorts of buttons on the top that do all sorts of this and that.
When you enter Layer, the current image goes into one slot (foreground?). You can open a matching dimension (and preferably registered) file, which goes into the other slot. You can swap which is Fg and which is Bg. You can go into Mask. Or you can create the mask before entering Layer. I believe the mask, however, is always tied to the original loaded file that you entered with. Or something like that.
The mask, custom (here, stars) or full, will be what gets blended, based on the type of blending chosen, into the combined result.
Fuzz helps blend the masked edges, and kernel radius imparts a bit of a softening blur. I have no idea why it's called kernel radius rather than blur. Probably some math thing. As an aside, Layer with a kernel radius blur is also very handy for singularities and blown pixels, say you saturated all your star cores to pure white, or M31's core is putting your eyes out. That sort of thing. A quick and easy targeted artifact repair.
Try it both ways, why not, but I would likely first give a shot to making the mask on the target (narrowband) image, sufficiently covering those stars and their surrounds (diffraction/glow) where you would like to get the color from the RGB file. As before, you will want those stretched sizes to be comparable in order for that to work.
Experiment.
There's a logic to combining RGB stars to NB, but there's some skill and artistry involved in pulling it off correctly. In a way it's NB Accenting but in reverse. You are RGB Accenting your SHO or HOO.
Fair warning, I've never been very good at it, especially against nebulous backgrounds. So I tend to just embrace my narrowband stars, particularly in SHO. HOO seems to need the help more.
Hope that's moderately useful. Ron and others who have done it more will probably have better and more concrete tips.
