The pattern is relatively simple, but I included some pictures for clarity on the last steps.

Gauge is not necessary, since the size of the musical note is determined by the weight of the yarn used.

The thicker the yarn, the bigger the note:

Fingering weight yarn = 3 1/4″ long (size of the green note in the above picture)
Sport weight yarn = 3 1/2″ long (size of the blue and orange notes)
Worsted weight yarn = 3 3/4″ long (size of the pink note)

Happy crocheting!