Follow the general instructions at https://github.com/simonschellaert/spotify2am .
This will export your Spotify playlist to csv and add the songs to your iTunes Library.
Notes:
For the proxy part I used https://mitmproxy.org/ instead of Charles. Charles isn't free.
mitmproxy is fairly easy to use, just don't forget to setup your HTTPS proxy server at 127.0.0.1 port 8080 (Settings -> Network -> Advanced -> Proxies).
If the insert_songs.py step doesn't work for you, try the one in this gist.
Note the TODO:REPLACE_THIS , you will get that info on mitmproxy when you add a new song from Apple Music to your Library.