I agree with the above. Being able to play the bass easily is no help really - it's not too hard to set that up and get comfortable with it. It's the co-ordination with fingers that is always the stumbling block.
IMO, the only way is
r-e-a-l s-l-o-w, bar by bar, beat by beat,
playing the whole pattern.
Eg, beat 1 might thumb alone. OK;
Beat 2 could be a pinch. OK again;
Beat 3, thumb again. But
between 3 and 4 you might get a finger stroke (on the "and of 3");
Beat 4, maybe thumb alone again.
Code:
BEATS: |1 . 2 . 3 and 4 . |
F F
T T T T
Naturally you need to keep those beats regular. But there's only ever 2 possibilities on any single beat: thumb only, or thumb and finger (pinch). Between the beats, it's only ever finger (could be any finger, btw). It's just a matter of stringing those simple elements together - and repeating each bar as often as it takes. Then moving to the next bar.... and then joining those two bars together... and moving on to bar 3...
That's how I did it myself - nearly 50 years ago now (age 16) - and I had it down in less than a year, maybe 6 months. (I mean after I first picked up a guitar.)