The MOD format is very complex.  This is a simplified description broken down by components.

1.  internal song title (20 bytes.)
2.  instrument info:  size, looping info, volume, etc. (30 bytes X 21 instruments)
3.  sequence length (1 byte)
4.  number of voices (1 byte)
5.  sequence table (128 bytes)
6.  format signature (4 bytes)
7.  pattern data:  actual notes (256 bytes X number of voices)
8.  instrument samples (size of each sample totaled)