Defining Milestones for A Song Arrangement Job


I am posting a job on a freelancer platform for a freelancer to arrange a 4-minute pop acoustic song ( for two singers) on fixed-price basis. I need to define milestones for the job. I will pay the arranger after completion of each milestone. I provide the arranger with the followings:

  1. two reference tracks
  2. Initial song arrangement (song is already organized into sections intro, verse, chorus etc).I have recorded the song on my iPhone with accordion, violin, the two singers)
  3. Chords-melody on a music staff

deliverables: 8 stereo or mono dry tracks ( stems) and initial mix of the arrangements, preferably as a single cubase project.

Of course, it depends on the arranger’s workflow. Here is one that seems to me more accurate for recording a song in studio.

I was wondering how I should split the arrangement into measurable milestones and deliverables? I am asking the song to be arranged in at most 3 months.

Any help would be greatly appreciated.