My routine so far (unless it's a simple / easy learner's piece such as from Palmer-Hughes)
- Listen to the melody, or listen to the familiar melody again, maybe a few alternate performances, and internalize
- Write out the notes in MuseScore / Dorico (or when possible, scan a piece, convert to MusicXML and edit to remove conversion errors)
- Determine the fingering separately, using an electronic keyboard, and add the fingering to the notation software
- Print out the "working version" of the sheet music, try playing with the accordion and confirm / make changes to the treble fingering
- Figure out the bass fingering, write on the printed sheets, later add to the notation software
- Keep attempting to play. Start rough, then refine.
- When it starts to resemble a listenable melody, REALLY go crazy and practice with a metronome so it all comes together