My trouble was getting moderately consistent timing and moving fingers in an order opposite to my inclination. See if this helps:
- place fingers on their associated string
- one by one, do a light pickoff without scooting the others much
- pick “upward”, like curling the finger just enough to get it to make sound where “upward” is generally toward your palm. This way the fingers are getting out of the way of the next string.
If you can manage that, then you are on your way. For the timing, I practiced by strumming my fingers on the desk or on my knee. I have been able to do it pinky to index since I can remember, but the roll required the other direction. Once I got that kinda working, the roll started to happen on the guitar. At that point I was still moving my wrist too much, so I did the individual pickoff until I had better control of the fingers. I practiced the first few bars of a song I like for weeks. I decided it needed a roll about every 2 bars and I like the way it sounds.