What you actually need to know to use ffmpeg:

  # convert "yoursourcefile" to "youroutputfile", using 4mbit/s xvid and 192kbit/s mp3
  ffmpeg -i yoursourcefile -vcodec libxvid -b 4000000 -acodec libmp3lame -ab 192000 youroutputfile

  # as above, but rescale it
  ffmpeg -i yoursourcefile -s 640x480 -vcodec libxvid -b 4000000 -acodec libmp3lame -ab 192000 youroutputfile

  # same as first, but use VBR with quality '8' (fixed quality setting, so bitrate varies to meet that quality setting)
  ffmpeg -i yoursourcefile -vcodec libxvid -qscale 8 -acodec libmp3lame -ab 192000 youroutputfile

  # same as first, but pass through the audio unchanged
  ffmpeg -i yoursourcefile -vcodec libxvid -b 4000000 -acodec copy youroutputfile

  # as above, but no audio
  ffmpeg -i yoursourcefile -vcodec libxvid -b 4000000 -an youroutputfile

  # similarly, audio but no video
  ffmpeg -i yoursourcefile -vn -acodec libmp3lame -ab 192000 youroutputfile

  # get a list of compression formats and output file types
  ffmpeg -formats


Breaking it down with each option / set of options separately (this is what the ffmpeg usage should actually look like):

Command:
    ffmpeg

Input file:
    -i yoursourcefile

Optional rescaling
    -s 640x480

Video conversion (choose one)
    -vcodec libxvid -b 4000000         (CBR bitrate=4000Mb/s)
    -vcodec libxvid -qscale 8          (VBR quality=8 -- arbitrary number, try different values)
    -vcodec copy
    -vn

Audio conversion (choose one)
    -acodec libmp3lame -ab 192000
    -acodec copy
    -an

Output file (this name appears on the line without an option before it)
    youroutputfile