It helps to know a little music theory.
- A song is a series of notes (duh), but those notes are grouped into measures (or bars)
- The measures are always the same length (very important!)
- Measures are split into 2, 4, 8 or 16 beats, and notes start on a beat. But not every beat; that would be boring
- The pattern of notes in a measure (a phrase) will repeat in subsequent measures
- There are (usually) at least 2 phrases (let’s call them A and B), many songs have 4 (A, B, C, D), some have more
- The phrases come in groups and repeat, e.g. AB AB ABCD AB AB ABCD
So the first task is to identify the measures. Listen to the song a few times and hopefully you will feel where each measure starts. There is often an especially prominent bass or drum beat.
- Create an action at the start of each measure for 20 or so measures
- Slow down the video and precisely place the first and last action
- Even out all the actions in between
- Copy all the actions and paste them repeatedly until the whole song has actions
- Position the last action precisely and even out all the actions again
- Iterate the above until you have an accurate sequence of actions starting at each measure
Now you are ready to identify the phrases
- play the first measure at half speed and determine where the notes are
- using dynamic injection (Open Funscripter option), split the measure into 2, 4, 8 or 16 (or a mixture) to have a bottom action at every note
- Notes will fall on a beat; do not nudge the actions manually
Repeat the above for the next measure. When you find a repeated phrase you can copy and paste from the previous occurrence. When you find a repeating sequence of phrases you can copy and paste all the measures.
That’s it! Questions welcome 
