I just finished the polishing of this Lednica-style helmet, but I can't find any detailed sources on how the Vikings attached horns to their helmets. Any advice?

Not.

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Nice, subtle April's fool.  Good one John.

Little known fact:  There were just holes in the helmets.  The horns grew out the Vikings heads. (Devilish)

However, once they got them on, they never took them off.  It has been proposed that they originally put them on as part of a coming of age ceremony and they just grew out through the holes.

 

I get nose guards. Boy do I get them, but every time I see one, I get the feeling that if I'd been a viking with one I would have been permanently crosseyed.

Nicely subtle horn joke, very low-key.