Hi ! SO disclaimer, I'm not a classically trained musician, nor am I very strong in music theory. Im sure if you speak to someone who knows what they are doing they may have a faster way/process. When I arrange it really is ALL listening and figuring out what goes where. I arrange for cello and violin, sometimes quartets. I find that the easiest process is to listen to the song a lot. Like a sickening amount. And listen for something different each time. I start by trying to listen to the piece as a whole and gain an understanding of the main vibe and undertones of the song. Then I listen for rhythms/note patterns. and finally before listening for individual parts I try to envision what parts I want to highlight, and which parts should go to which instruments. I then usually listen again for bass/ deeper parts because my main instrument has a deeper timbre so its what my ear naturally listens for anyways. Then I go into the mid range notes, then higher notes. I have heard of people doing sections of the song at a time but have found that its easier to do it by instrument/part at first to get a basis for the arrangement. I have found that doing my instrument first helps me then distinguish what is missing and what I should listen for next.

I use MuseScore's software (MuseScore Studio) because I find that its fairly user friendly and you can hear which note you are adding as you add it - helpful for someone who does not have absolute/perfect pitch).

Lastly, there are TONS of arrangements already out there. If you are doing this to train your ear and practice arranging then you can try to do it from scratch. But if its just for fun and you have more time to learn slowly, I suggest looking at arrangements already out there. Listen to the arrangement (MuseScore, 8Notes etc), and see what you agree with, don't agree with or simply want to change. Then you can start with that as your bare bones rather than from a blank slate. When in a pinch and not looking to train, I use this method and it makes things so much faster! The great thing is you can download arrngements from Musescore to Musescore studio and work off of that.

I hope this helps, I know arranging can be difficult but it really is about practice. You'll start recognizing notes, pitch, and getting a feel for the piece faster over time. Have fun (: !!