Installing ffmpeg from dmg and then running from a terminal, is there a better way? From here: https://ffmpeg.org/download.html#build-mac I chose static builds 64-bit which too me to https://evermeet.cx/ffmpeg/

I then downloaded this:


  


and opened it using this:


  


which gave me this:

Last login: Wed Jun  6 09:41:56 on ttys000
david-selfs-Air:~ david$ /Volumes/FFmpeg\ 91200-g1616b1be5a/ffmpeg ; exit;
ffmpeg version N-91200-g1616b1be5a-tessus Copyright (c) 2000-2018 the FFmpeg developers
  built with Apple LLVM version 9.1.0 (clang-902.0.39.1)
  configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-version3 --pkg-config-flags=--static --disable-ffplay
  libavutil      56. 18.102 / 56. 18.102
  libavcodec     58. 19.104 / 58. 19.104
  libavformat    58. 17.100 / 58. 17.100
  libavdevice    58.  4.100 / 58.  4.100
  libavfilter     7. 24.100 /  7. 24.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...

Use -h to get full help or, even better, run 'man ffmpeg'
logout
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.
Deleting expired sessions...7 completed.

[Process completed]


However when I open a terminal and try ffmpeg or man ffmpeg or even ffmpeg -i filename.mp4 -vn filename.wav(from here) all I get is 


  -bash: ffmpeg: command not found


I must be close, is there a place where I now need to define a path for ffmpeg? I don't know how to do that, or what path to use. 

Is there a better way?
You are supposed to copy the program from the disk image to your local folders. The usual place for these things is /usr/local/bin

So:

sudo mkdir -p /usr/local/bin
sudo cp /Volumes/FFmpeg\ 91200-g1616b1be5a/ffmpeg /usr/local/bin/
sudo chmod +x /usr/local/bin/ffmpeg


and all should be good to go. Disabling SIP not required.

No need for homebrew or any other package handler to install a precompiled binary. You do need to read up on basic UNIX though.