If the ID3v2 tag exists in file, then first four bytes always are (49h 44h 33h 03h). All sections are presented in random order. There is a names of sections:
TENC - 'Encoded by'
WXXX - 'URL'
TCOP - 'Copyright'
TOPE - 'Original Artist'
TCOM - 'Composer'
TCON - 'Genre'
COMM - 'Comment'
TYER - 'Year'
TALB - 'Album'
TPE1 - 'Artist'
TIT2 - 'Title'
In mp3 file starting from offset (0Ah) follows described sections. For example: 'TIT2' then the length of string in seven bytes (00h 00h 00h 0Dh 00h 00h 00h) - 13-1 chars, then string. Then again the name of section, length and string... All sections are described!
If no string in section, the length is (00h 00h 00h 01h 00h 00h 00h) and then name of next section.