I had to start very, very slow. Like quarter notes DWPS rest strokes) at 60 bpm. The metronome was essential here. I rarely use one otherwise.
Once that feels consistent and smooth, bump up the metronome 2 to 5 bpm and continue the game.
Every once in a while, jump up to sloppy 16ths at 160 bpm or so. See how clean you can get it in a few minutes, then back to slow and deliberate. Try to marry the two speeds together until you have the feel memorized.
I did 20 minutes to an hour, once per day. After a week it was usable after a short warmup. After a month it was second nature, and I stopped practicing it in isolation.
At very slow tempos, don’t slow down the pickstrokes themselves, just put space between them. Each pickstroke should feel like a distinct event, they shouldn’t smear together. This helps you memorize the feeling of the individual strokes, which is necessary before you can start chunking, IMO.