The hammered dulcimer is a tricky instrument to play and even weirder to write for. The layout of the notes, based in a few different modes and usually related horizontally in 5ths, is far more confusing than most instruments. There are plenty of guides to this online, but I wanted to make a chart that illustrates where the pitches live on the staff:
Keep in mind that this is for a dulcimer with a 16-15 tuning, meaning 16 treble strings and 15 bass strings. The right side of the bass bridge will only wring harmonics and is in most cases not struck (though I could get into that at a different time). The range of the instruments vary, and it’s ideal to have an idea of who will be playing your music so that you can compose accordingly. That being said, 16-15 is a fairly standard range.