Introduction
There are two things to work on during this warmup exercise:
Practice picking individual strings using the 3-finger assignment with the right hand: Thumb on G & C strings, Index on the E string, Middle on the A string.
When incorporating a strum between picking, practice using your thumb to brush across the strings.
As you get comfortable with the finger assignment, experiment with your own patterns for plucking the individual strings.
Practice holding chord inversions using the left index finger to barre. Develop your sustain by holding the chord shape for the duration of the finger picking pattern.