After the Portarlington Festival, I started to arrange O'Carolan's Blind Mary (which we'd played in a workshop session) for two guitars. It hadn't occurred to me until this point that I've never spoken to other arrangers of traditional music about their sheet music process. For me, this is completely different to the aural process I'd use to devise a ‘live’ accompaniment, because when arranging for sheet music, I work alone. Here's how I'd approach a new arrangement:
First, I start with the instrument I want to write for. My guitars all have their own characteristics. One is classical; my favourite steel string has an even tone anywhere on the neck; the other steel string has a lovely mid-range, but goes slightly out of tune if I try to play too high up the neck, particularly the bass notes. I also have a silent guitar so that I can work in my music corner while the family watch TV, using headphones so that I don't disturb them. The problem is, I find that if I use the silent guitar to write or arrange, then once swapped to the ‘proper’ acoustic guitar, the music I've written never sounds the same. I've only just realised (after too many years) that whichever instrument I use will shape the arrangement I'm working on. I'm favouring the cutaway guitar at the moment, because I can access notes higher up the neck. Because the tuning is more stable with this particular instrument, I can be freer with alternative tunings and a wider range. All this affects the end result.
Second, I notate the melody (always in tab if the guitar is in an alternative tuning). Then, I play around to see what chord or inversion I'll want underneath. This is the part I enjoy most about arranging. Most listeners expect traditional music to be reasonably consonant rather than completely experimental, so I'm informed by other arrangements in the same genre. There's a modern fashion for messing with rhythms in traditional music - this is great fun, especially in dance pieces which are played fast. I'll admit that I do follow some classical rules. But, I wouldn't do this to the extent of spoiling a folk arrangement which needs a drone-style accompaniment in consecutive 5ths, say. My easy Christmas carols have a few ‘hidden’ consecutives, but the alternatives would've made the pieces too tricky for the level of player they're aimed at, and the arrangements work thanks to the surrounding harmony.
Third, once I've chosen the chords, I fill out the harmony just a little (nothing too dense), and then write the second guitar part. This is where it gets interesting, because I don't use a recording studio to test the harmonies. Before mobile phone days, I used to sit at the piano (or use the guitar) and bash out both parts simultaneously, which can be tricky. I'd then use a live rehearsal (with as many musicians as required - some of my arrangements are for trio or small ensemble) to test the parts. Because I don't always have access to other musicians, for duets, I've recently started using my mobile phone to get a rough recording of part one. I use the phone to play back part one whilst playing part two (live) alongside it. I find this method more useful than the playback on Musescore, because on a real instrument you can get away with some things which don't sound great on the computer playback, and vice versa.
Fourth, I complete the notation. The fine-tuning to arrive at a final version takes a while, because it's always good to experiment with ornamentation or with notes which might be played in different places to give a different effect. This is possibly a unique quirk of fingerboard strings; on instruments like harp and piano, a note can only be played in one specific place rather than having alternatives which sound subtly different.
If I don't write the music down, I know I'll forget it!