Very tricky...

George Harrison (of the Beatles) was taken to the cleaner by The Chiffons' lawyers because they claimed he pinched the backing of 'He's So Fine' and put a different tune and words over the top to make 'My Sweet Lord'... not exactly the most innovative of chord sequences...

If ever there was a travesty of justice... libel and slander laws prevent me from suggesting what I think...!!

You need to be able to demonstrate that you have not plagiarised anyone else's work to produce your original, which basically means the actual music isn't the thing you're copyrighting, but the musical arrangement as a whole.

I'm not an expert in these matters by any means, but that is how I've seen it happen in the past. Over the past few years I've logged music played on radio stations for PRS purposes, and it seems that way in general:

The usual wording is 'Trad. arr. (name)', or similar - you are acknowledging the source and adding your name.

I would suggest that as a harper you're in a good position, because your arrangements will be tempered by your playing style to start with.

I'm sure you'll get lots of helpful advice in this thread, plus links to other threads!

;o)