One note per token. Letter, octave, then a slash and how long it lasts.
C4 middle C · F#5 sharp ·
Bb4 flat · r rest
/w whole · /h half ·
/q quarter · /e eighth · /s sixteenth.
Add a dot: /h.
Leave the length off and it stays whatever the last note was, starting at a quarter.
Headers, one per line at the top:
title: composer: time: 3/4
tempo: 90