Hi, for the most part, chords between lines are 4 beats to the bar (unless it’s 6:8 time, but there are few of those in the book). In 3 Little Birds, there are 4 strums per bar for the intro, there are 4 bars so 16 strums total. When I learned this, I use all down strums, 1 per each beat.
In the choruses and verses, there are no bar lines shown but 2 chords per line, Justin sort of touches on why there are no vertical lines in the Introduction of the first songbook. Each of those chords on each line is its own bar, so there are 2 bars for each line, or 8 total downstrums.
How I played this, was 4 downstrums for the first chord and as I switched to the next chord, another 4 downstrums for it, and so on.
So, for the first line you’d sing the first word, then on “worry” start strumming the A chord for 4 downstrums, “thing” starts the next 4 downstrums on chord A again, till you get to “very” on line 2 where you start 4 downstums on D, and so on. Write in the vertical lines yourself to make it clearer.
I don’t recommend using Old Faithful for this, not all songs sound good with it. I never could get the offbeat Reggae strumming figured out, so don’t worry about it. This song sounds just fine with 4 downstrums per bar.
I hope I haven’t confused you more.