Download the demucs app:
https://github.com/CarlGao4/Demucs-Gui/releases
Open the app and press "Load".
Click on the "File queue" tab.
Drag and drop your mp3 file into the app window.
Click on "Start separation".
Once finished, you'll find a folder in the same directory as your mp3 file.
This folder will contain separate .flac music files for the vocals, drums, guitar, bass etc.
You can then recombine the song, omitting the vocals, using the following terminal command:
brew install ffmpeg
ffmpeg -i other.flac -i bass.flac -i drums.flac -filter_complex "[0:a][1:a][2:a]amerge=inputs=3[aout]" -map "[aout]" -ac 2 -c:a libmp3lame instrumental.mp3
Note
Replace/update the -i other.flac, -i bass.flac etc, depending on what parts you want put back together.
The ffmpeg command will generate an instrumental.mp3 file.