Had to watch the video to figure out the left hand. Skip to 2:50. Good pinky workout.
Edit: FWIW, I alternate (cross) pick it, too. The deceptive part was that I couldn’t make the right hand groove correctly without optimizing the left hand first. The trick for me was realizing that I didn’t have to hold my ring finger down during the trill, so I could use my ring finger for the trill. That’s why the video was helpful. At first I was trying to hold all the notes while doing the trill with my remaining finger, and there wasn’t enough room so the groove was suffering.
I agree with StickyFingers. Make sure you can do the right hand effortlessly first. I suggest just holding the first chord down to start, and skipping the trill. Repeat on one chord until the right hand is easy. Then add the trill. (To make the groove really snap, emphasize the first note of the trill, which is easy if your chunks start on the 1st and 3rd beats. Also cut the root note off at exactly the same time as the start of the trill, which took a few reps for me.) Then start switching chords. At least that’s what I did.
Hope that helps.